The description of the CAD is in the RB, pg 122. You can also find the "allied" detachment, which is somewhat ironically NOT the only way to include allies, which can be kind of misleading.
You can take as many CAD as you have points for. Since I like playing with lots of the "Elite" options from the Space Marine Codex, I often play a Double CAD with 2x HQ, 4x Troops, 4-6 Elites, and sometimes a Heavy Support or Fast Attack option. That's a Battleforged list.
Many codices have their own "Faction Specific" Detachments, such as the Baal Strike Force, included in the BA Book. Only BA's can use the BSF detachment. Instead of gaining ObSec, they gain bonuses when they're charging. You could have a BSF and a CAD, as a legal army. The models from BSF gain the charge bonuses, the CAD gain ObSec on their Troops. But you don't get both, just the models from each detachment get their own rules.
You can add Formations, or run lists composed entirely of Formations. These tend to have specific unit / model restrictions, and tend to have more powerful benefits, or alternatively allow highly skewed lists to be built.
The Archangels Strike Force, from the Shield of Baal - Exterminatus campaign supplement [came in the BA vs Nids box, I think] allows you to take a Formation of 1-2 HQ, and 2-16 Elites, specifically Terminators, Vanguard, Sternguard, and Furiosos. So if you want to rain down 60 Terminators in Squads of 5, in a Battlforged list? This is for you.
That supplement also has 12 or so more formations... including the Ever-Popular Codex: Taxi-Service, that lets you take an 1 HQ [cheap Tech] 1 Troop [5 scouts] and 6 Empty Drop Pods. Which you can Battle-Brother and allow things like IG Vets to use Drop Pods instead of Chimera, which is pretty be-atchin', as it turns out.
You could google "Blood Angel Formations" and at the very least, you'd know the names of the formations to source out. Same deal with Nids, though you might want to focus initially on Shield of Baal - Leviathan which allows some nifty formations. Something like 3 Flyrants + 2 Spore mines [At the cost of a couple Marines.] which allows you to basically play something like 5 Flyrants in 1500 points? Something like that.
