ESP,
The main thing to remember is this:
1. There are main different brands and bullet weight of ammo. Each one will usually have small differnt points of impact, which will necessitate adjustment of sights.
2. There are two different types of ignition (primer) systems, Berdaan and Boxer. Berdan is sometimes corrosive, has a longer storage life, is not reloadable, and is usually dirtier. Boxer is not corrosive,cleaner,reloadable, and usually (but not always) seen in non-military ammo. Both types work fine in any AR15
3. Some ammo has a steel case, mostly Russian brands such as Wolf. These cases are also usually coated with a laquer finish and are Berdan primed to give them very long shelf life. Some AR15 (like mine and many others) have problems with this ammo. The reason is because as the barrel heats up, the laquer gets "melted" onto the chamber,bolt, and carrier of the AR15. This often results in jams and feeding problems. It really is best to stay away from this ammo for AR, but it works great in AK47's and SKS rifles.
If you really want to learn more, I suggest you look for a used cheap reloading manual at the next gun show. These explain everything.
Sorry for the long post, but I hope it helps.
sgtar15