I am simply writing this for the new Eldar players and those who are just looking to start out with an army and have no clue as to where to go. To go with this I am simply going to cover how units are designed to work and tips on how they are usually played in 6th edition. As a sort of notice I am merely a novice at the game, but I have scoured the known forums containing 40k and pooled together all the information to make this article so that you don't have to go through the information overload that I went through. More experienced players may add where they see fit.
Gotta start some where so we might as well start with the leader(s) of your army since they will greatly impact how your army is going to function.
Farseer + Warlocks
Now the Eldar are known to be the most powerful psychers in the realm and everyone pretty much agrees with this. In order to walk the walk, the Eldar have the unit called the Farseer and is the most commonly used HQ choice the Eldar will see. This is because they can cast up to 2 abilities per turn and the choices they have each have their own function. Doom, Fortune and Guide are spells that either affect the enemy or your squad, but always benefit you and are the most used spells out of those available. There are 2 more spells and they are the 'shooting' spells called Eldritch Sotrm and Mind War. They are situational spells that most generally don't look to because the other 3 benefit them so much more. The Farseer should never be alone and in a squad that he will support with his abilities.
Warlocks are a unique kind of psycher where they have 4 spells: Embolen ( makes squad fearless), Enhance( benefits melee groups), Destructor( flame template), and Conceal ( helps add cover saves to the squad). They can be added to certain squads, as long as a Farseer unit takes up the hq spot, or they can form their own squad and form a seer council, but that is something I will not cover and leave you to research up on. These guys mainly go where Farseers can't and support units as needed.
This guy is one bad mother Psycher. He has all the doodads you can put on Farseer and he is able to cast 3 spells in one turn, so long as you are not locked into combat with him. Now for people that know about the powers, do the math when you can potentially cast Fortune (reroll armor, cover, invul saves) twice and guide/doom a squad so that the only thing you hear is the agonizing screams of your opponents and the laughter of your troops as they cut down the enemy while shots bounce off of them (dramatization). Eldrad is a must have in armies larger or at 1000 points because he can only benefit an army.
This is the brute of the Eldar HQ. He may not look pretty, but you get within his reach then you might as well consider yourself dead. Few people are able to go toe to toe with this beast and even then few can match his power. He also makes friendly squads within 12" of him have fearless so your squads are going to be that harder to ignore/kill. This guy is great in an army list that contains foot eldar and very few mech if any. This guy is Chuck Norris in that he is the lone ranger and will be by himself, but never far away from a squad that can back him up if the dice rolls are never in his favor. If you are defending then keep him out of LOS so he can pop out and bash on who ever dares to enter his domain, but if you are on the assault then just send him into the thick of the enemy and watch them all run away in fear!
In other words the specialist HQ that can be put in any squad and be able to lead it. The effectiveness is another question, but the man can hold his own and improve any squad. He really isn't used much because most Eldar generals prefer Eldrad and Farseers to bolster their army. You will mianly put the Autarch in a seer council to give it a more melee presence or you can stick him with a melee footslogging squad and run along side them and reap carnage as the squad goes into enemy lines.
This guy is pure monstrosity. Another melee HQ choice, but he can fell several units with just a swipe of his Lance. If he ever gets stuck in a sticky situation and there is no help then does he have a surprise for the enemy! Prince Yriel packs a nuke in his eye and is able to unleash it only once a game, but o what a pretty site it is when it goes off. It's like s magician putting a cape over the squad assaulting him and when he pulls away the cloak the squad is no longer there. You would not like Yriel when he is angry...or maybe you will since he is on your side. Play him like you would the Avatar except he can join a squad and make it a real threat.
These guys are the first or most famous person of their aspect. They give their aspects Exarch powers so no need to buy that Exarch and the stats for the Lords are just so mind boggling that it contradicts the conception of the fragility of the Eldar. Pheonix lords see a lot of play when the army list is fit for 1750+ points and not so much in 1500, but never below since they just cost too much. These guys are not invincible so don't send their squads to their deaths. They are still Eldar so you still need to have tactics as your primary weapon.
The units that become the arms of your army.
These guys are slow for Eldar, but for every con there is a pro. They get a 3+ save and a S 4 which is very unlike Eldar who are supposed to be fragile and weak. They are also the melee force and get many attacks to slice into mobs and do a little crowd control. Any squad lower than 10 is really not a squad, these guys need the numbers because they can be delicious targets for the opponents squad killers.
The anti tank unit. Put them within 6" of a tank and you can just say it is wrecked/exploded. A must have to be a part of the army unless you have heavy support units that can take over the role of the Fire Dragon. Strongly advised to use a wave serpent to carry them and drop em next to a tank. You don't need more that 6 of these guys and only 5 if you take the Exarch. Just put em in a WS, dump them next to a tank, then watch the fire works. Anything melta wrecks armor units (tanks and fliers) so keep that in mind when you face other meltas as well.
They were extremely sought after in 5th because of their capability to jump out a WS kill and then jump back in. Now people say that since this is done with, that banshees are no longer useful. They are still useful and can cut through most armor and are used to go after the higher armor units with their power weapons to cut through them and decimate. If you do field these units then make them the defensive team and not attacking.
These guys are considered to be the most powerful ( and expensive) of the Eldar Elite melee units. Simply because they have a weapon that gives rending and it basically makes the unit go in a poof of blood and gore. With out the rending the Harlequins are pointless to have, but a unit in the troupe can be upgraded to a shadowseer which helps hide the squad from harmful fire, a death Jester which is the only ranged harlequin and the Troupe Master, but you really don't need him and should only take him if you have points left over to spend. Always go into cover when travelling up field with these guys or keep them in cover if your on the defensive and protect objectives and what not.
These guys are the walking wall of Eldar. They can take wounds all day and make saves left and right. Only problem is that their weapon of obliteration can only go 12" so it usually takes a while for them to start wreaking havoc. Unfortunately they suffer from Wraith Sight and if they are not within 6' of a Pscher unit then they have the chance to sit around and do nothing. To solve that problem, upgrade a warlock to a spirit seer and then the entire squad becomes a troop choice and that makes most opponents shudder. Always have 10 of them in a squad and never anything smaller.
The Wave Serpent is going to be you method of zipping squads of people around to do what ever your heart desires because the Eldar say " O you want me to go to the other side of the board? Done." This should really be the only thing hold units because it is cheaper than the other option and faster.
the Skeleton or the Backbone of the army. These are the guys that can make or break and army. Always have 3 squads of these guys since your troops will be targeted if the enemy is ever given the chance.
By far the best troop choice out there. They are capable of dealing 3 attacks at 18" away ( at the cost of not firing next turn) or deal their normal 2 shots. They aren't so bad at melee either and can hold their own, so long as the Exarch is prepared for close combat as well. After that they can even go on their own or go into a WS and shoot whole squads to bits. be wary if you make them in small squads. An enemy will look at them as a snack so be wise if you go with a squad of 5, but other than that a squad of 10 will fire 20 shots and that is without the exarch. They can be used to hold objectives in the backfield, attacking range unit, or a enemy objective contester. A very versatile unit.
The runts of the litter, but they are not that horrible as people say. They take a wep platform that doesn't have to snap fire if it moves and can take any wep that can be loaded onto a WS. So you get your anti tank or anti foot squad and will be able to help out your more specialized units with what ever weapon they put on the platform. These guys are what will stay on back field objectives and if there are none, then they are staying as far as possible out of battle just protecting your back side in case any thing deepstrikes from behind.
The snipers of Eldar. If they catch the enemy in their sights then the squad is out of luck and will say good bye to their fellow comrades. They can stop whole advances by foot armies thanks to their pinning and are hard to kill since they improve the cover around them. Now if you upgrade them to pathfinders then they are even harder to kill and can pin squads easier than their ranger counterparts. These units can be taken in a squad of 5 or 10. A squad of 5 would be for helping to keep units pinned while a squad of 10 is more looking to wipe out anyone who dares to come into their LOS.
Guardian Jet Bikes
The fastest troop choice and used to deny or take objectives at the end of a game. After that they are not all that useful unless a seer council is present on the board to boost the jetbike squad to a status that can threaten an army.
Now seeing how Eldar is already fast and that the group is the legs of Eldar this should spell trouble for any slow army.
Warp spiders, if used right, can be the unit that lends a helping hand or can be the guerrilla force that harasses units and withers them down to the point where you have other units be the mop up crew. They are great at jumping around and shooting into the rears of tanks that normally only have AV10 and with the volume of shots that they put out it is hard to believe that a tank will survive the brutal assault they give. How many you put into the squad actually defines what the unit is going to do.
The 5 man squads can pop a few units here and there along with a separate 5 man squad wreaking more havoc. Having a 7man+ squad means your looking to annihilate a flank or at least slow it down by killing a squad or two. You don't want to use guerrilla warfare with this group because you have the potential to beat a squad down and possibly force it to run. If not then you can jump out of range and wait to strike again and force the unit to focus on something other than what they were originally going for.
So Warp Spiders could be either the annoying fly that you can never really hit or the snake in the hole, just waiting to strike and swallow up a unit whole.
Swooping Hawks are one of the more 'controversial' units of the group. They have potential to wipe out a good chunk of blobs, but outside of that is where people tend to say their usefulness runs out. They can be used as a last minute objective contester, line breaker, tank buster, and/or mob control. The first two are obvious from the deepstrike ability they have with their wings, but the real issue lines up with living long enough to get to a tank and wreck/explode it. They have the capability to, but they are so fragile that most of the time they never live to see another day once they are done flying back into the sky. They can control mobs, like gaunts, with their grenade packs and dwindling down so that your more shooty units can focus on the scary scary ones or they can come down and the exarch can shoot a gun that shoots 6 shots on its own. Right there that makes it a very interesting thing to invest in, but take into account the fragility of them so you better shoot and then run away since they follow the jump pack rule, that is if you allow them to stay on the field for more than just the movement phase.
These guys are pretty much a Jet Bike that can fire a guardian wep platform, but my god are they fast. Do not over look these units for their speed can turn the tides of war.
These guys take jousting, but make it so that if they hit some one with their lance they are not getting back up. They even have the probability to cut through medium armor tanks (AV12) and slice em up into nice little pieces. O ya and these buggers are fast as well.
Well we have bones, a head, arms and legs, but we can't move unless we got our muscle!
I don't know much about these units other than that they can really make your opponent worry as they create havoc and whittle your enemies forces down to something insignificant. Have them in a squad of 5 with an exarch and then any unit cowering behind cover can potentially become part of the cover...well at least their lifeless body will be.
Support Weapons are one of the weapons that Eldar people are looking to see if they get changed in the new rumored codex. As for now they are not a common unit and are not considered to be competitive. What is a must though is that they should have a body guard at all times.
D-Cannons: This is a great weapon for its point cost except the only problem with it is it's range. At 24" they can be hit at from a few weapons, so target priorities are needed when fielding this. Another thing to consider is that since it is a heavy with a template, you can't even snap fire if you move it. So you should think carefully if your going to use it to maintain dominance over the middle of the board by advancing or will you allow the enemy to come to you and open up an array of fire that can send entire squads into the warp for them to float aimlessly for all time. a squad of 2 is minimum and 3 in a squad just sucks up entire squads if you focus on one.
Vibro Cannons: This choice has a lot of questions revolving around it. To not address them atm I will just talk about what they can do. First off they don't need TLOS to hit people, second any unit the 36" line hits ( yes even yours) the unit has to roll d6 and that is how many saves the unit has to make and a potential pinning check, but with tanks and skimmers they automatically deal a glancing hit. Now another weird thing they can do is be able to hit units in combat. Wait what? Yes they are probably the only model that can hit units while they are in combat per the FAQ. Now all you have to do is make sure you don't line up with your own unit so that the advantage is all yours! A squad of 2, no more and no less, is recommended since 1 is just too few and 3 is too pricey.
Shadow Weavers: These are probably the silent kid in the corner that no one pays attention to. Mainly because there are so many things that overshadow it, but the few things it has going for it is the 48' range and ( I believe) it has multiple S6 barrage.
Notice how biased I am with the first two :P.
A good tank option and really the only viable tank option, but wait its a fast skimmer as well so away it goes! Do not use this for a transport because your going to want to sit back and shoot at enemy tanks with its permanent weapon and/or a BL or EML.
Fire Prism + Night Spinner
Now the Fire Prism can be used to take out medium tanks and crowd control, but not really be used to kill something like a landraider. Leave that to your Fragons or other ant tank units. The night spinner though is the ultimate crowd control weapon. It is known to cut down blobs in half because of the large blast it throws down and the ability to rend, barrage, and put the unit in difficult terrain which may cause the squad to lose even more units! The fire prism should have a partner on the field or else it isn't worth to have the night spinner can stand alone though.
This behemoth is what you would use to take out tanks by throwing a BL on its shoulder and when you put a sword in it's hands it can throw half a squad into the air! This is a guy that sees a lot of play and is worth every point. You can have one or two of these guys but not 3. You need versatility not a field of monstrous creatures. The 2 lords should also have the same load out because if one falls then there is another one that they have to concentrate on, but if they are not the same then they just have to kill the one that is hurting him/her the most at that moment.
These guys are cheap and if you throw a guide onto 3 of these guys that have 2 SL ea we are talking about 24 shots that have the ability to reroll if they don't hit. Pretty devastating and can turn a whole blob squad into something like a tattered rag on a flag pole. Another viable option to take up your Heavy Slot.
That's all for reading and hopefully you chose Eldar or have a better understanding of them now to start making lists that follow your game style.