Yes, that was a really simple response.
Sadly it's also false on a lot of points.
'Allied Detachment' has nothing to do with 'allies'. (Yes, that sounds weird. But in
40k it are different things)
A 'battleforged' army has nothing to do with 1
HQ, 2 Troops, etc.
I'll explain it correctly:
When building an army you can pick between unbound and battleforged.
Unbound = Any number of Formations and any number of loose units.
Battleforged = Any number of Formations and any number of Detachments.
This means you cannot mix whatever you want. You cannot take a Formation, a Detachment and some loose units.
Warlord = The 'general' of your army, the detachment that contains him is what we call the Primary Detachment.
You can also have a Warlord in an Unbound army, but I will ignore that for a moment.
A Detachment means that you take a specific amount of slots for everything and you fill those with units from a Faction.
A "Combined Arms Detachment" requires 1
HQ and 2 Troops and you can fill it out with other stuff (read the
BRB to see what you can add).
You also have an "Inquisitional Detachment" that requires only one
HQ at minimum; but you are also limited to only units from the Inquisition Codex.
Then we have the "Allied Detachment", which has a minimum of 1
HQ and one Troops. But this Detachment also has a restriction: It may not contain your Warlord and MUST be from another Faction as your Primary Detachment.
We have a lot of Detachments and most of them are restricted to certain Factions (like: This Detachment can only contain Orks), the "
CAD" and the "Allied Detachments" are one of the few that can be taken by any Faction.
So now you probably wonder: "Why care about Detachments, that seems like a lot of trouble?"
Well, Detachments don't just have restrictions and limitations. They also have bonuses!
We have Detachments that give your units Deep Strike, super-scoring, T1 Deep Strike, etc, etc.
And most of the time you already picked units that fit in a Detachment, so why not make it a real Detachment and gain some small bonuses!
Now you are probably wondering the same about Formations
Formations are basically Detachments with stricter limitations and bigger bonuses.
The Allies-chart is a table in the
BRB.
Every Faction can play together now, but there are special rules for many combinations.
Ultramarines with Imperial Guard are best buddies, so there is hardly an issue.
But how about Ultramarines with Tyranids? You better keep those away from each other!
The Allies-section in the book tells you exactly how those armies act if you play them together to defeat an opponent.
For a new player (to 7th edition) I have two pieces of advice:
1)
http://www.dakkadakka.com/dakkaforum/posts/list/0/617215.page
Go to this thread and try out Battlescribe, it's software for
pc, mac, iOS and Android that lets you build an army.
When you pick too many units for a slot like
HQ, it will warn you and it will do that if you pick too few.
2) Just take your books and look at the Detachments you can pick from.
Let's say you play Dark Eldar!
You take the Realspace Raiders Detachment because you like the bonus since it gives you additional cover.
You then add at least 1
HQ, 2 Troops and 1 Fast Attack to your army.
Then you add any additional amount of units as long as the Detachment allows it.
You now have an army to play with!