ok so, while everyone is just throwing out names, ill give you all armies and what they do in a short sentence or 2
lets start with the hord type armies first
orks: these fellows are all about hords all of their stuff is cheap, but most of them die very easy, they usually play 1 of 4 ways, get in to close combat and spam them with units till their faces get ripped off by 30 orks in 1 squad, shooty where you will miss 80% of the shots but blot out the sun with your bullets, air craft oriented ( with the help of forge world ) where almost everything can be in the air, or mech and tank oriented ( with the help of forge world ) to spam so many tanks and walkers that they wont be able to take it all on
tyranids : these aliens are pretty bad, don't get me wrong they can be a force to be wreken with if they are doing their stuff well, but they are painfully easy to kill, once you know what to kill, unless that thing you have to kill becomes impossible to kill, they have 'gaunt spam, monstrous creature spam, flying creature spam, or mix that just have big monsters that spawn little monsters
ig or imperial guard : forge world loves these guys, and they are probably the highest tier army in the game, their infantry are horrible but spamable where in a tournament game you can have 200+ infantry running around but they die pathetically easy, their close combat is pretty bad, and they are litteraly given flashlights as weapons but youll have so many that it doesn't matter, they also have some of the best tanks and fliers in the game, and probably have more units then any other army in the game ( thanks to forge world )
( starting to leave the hord zone but every army can go hord if you build it right, all depends on your play style )
necrons : necrons are space robot egyption zombies with green lightning guns that arise from the dead when you kill them, do i need to say more. well yea i should, necrons can spam infantry to no tomarow and their basic gun can kill their strongest tank ( if your lucky enough ) their vehicals are a mixed back as they don't have alot of them, and their flyers are really good too, but keep in mind this is a close range shooty army that will die very easy if they get into close combat, so be careful
eldar : dont pick this army, if you like it wait till their book comes out in a month or 2, but as of now they are pretty bad ( still in 4th while everyone else is in 5th or 6th ( exept for orks but they still do really well ) they are specialized units that do 1 thing and do it well sometimes, the forge world army list for eldar is pretty good but as of now normal eldar are kinda bad
dark eldar: have alot of units, move at the speed of sound, hit like a tank running over an anthill, but have papermache + cardboard for armor, this can be hord but then again it can also not be hord depending on how you bulid it but in this army you will have alot of " boats " for transports as they move really really fast as they soar across the board, thats how they usualy play too, few air units, alot of " boats " filled with infantry and shoot everything or get into combat with everything till nothing is left so that they dont get shot to pieces
space marines: these guys are the best of the best infantry on the board ( well the most basic guys any way ) and they can take a beating ( unless your unlucky ) as well as give a beating ( unless your unlucky ), space marines ( and their equivalents ) are the most played army, seeing as they can be played almost anyway you want and are the easyest army to use seeing as they are all round with nothing bad
chaos space marines: these guys just like space marines can be anything, and the best part is, all chaos armys are so custimisable that no one can counter your list unless they either get lucky or they know what you are using, this army can go horde, elites, tank spam , air spam, mix and match and anything you can think of they can do it, also in short they are space marines but demonized
chaos daemons : chaos daemons are more infantry relyant and are spamable, but like chaos they cannot be easily countered unless they bring alot of anti infantry because thats mostly what this entire book is, this army is all rounded, nothing game breaking, but nothing so bad you will loose every game, its ok, but not the book you would want to start if your new
sisters of battle: the single most hardest army to do because it is so hard to find the models and the book for this army, they are bad as well because that they almost never get updated and hardly anyone plays them, but that is also a good thing, no one knows what to expect of sisters of battle because not many people know what does what, to the actual army it self it is space marines mix with infantry spamablity of imperial guard , with a unique rule that you " god " emperor helps you spiritual, unless you want to put the effort into finding these then go ahead play them, they are fun but wont win alot of games
tau: these guys spam units, bunker down in cover, and shoot till the entire enamy bored edge is gone, these guys are the quintessential shooty army, they also have " suits " ( or mechs ) that are custimasable to your desire as you like, that's about it the only way to play them is either long range shooting or close range shooting, just keep in mind that you WILL DIE in close combat, so don't let them get close
now we come to the part where i explane space marine and their equivalents, because space marines have about 4 other books for the space marine army, so if your going space marine, give these books a look
blood angels: are the " fast " and close combat oriented space marine army, they got nerfed pretty bad in 6th edition witch made them pretty bad now, you will still win games, but have a hard time doing it
dark angels: where blood is
cc the dark angels are the shooty army, that focuses on super buff infantry called terminators, or space marines on bikes, these guys are ok
space wolves : the army is meant to be close combat oriented but is actually better in shooting, they also got nerfed but because of some broken things they have they are still really good ( when you fight a space wolf player youll know what i mean )
last one
grey knights: these guys used to be over powered, but because of 6th edition they became "ok" greyknights are the close combat and close range shooting army that will kill anything it gets close to, but the problem is that they have to get there
and that is all of them, hopefully that helps if you need questions just ask and sorry for me getting lazy tords the end