ARs are the tinkertoys of the rifle world, so here's a basic, breakdown, which can't possibly be complete.
For all intents and purposes, you can think of them as rifles in 3 segments:
Upper Receiver
Lower Receiver
Buttstock
Each of these come configured in "pre & post" ban configurations. All "pre-ban" configs must have been assembled into a complete rifle prior to the magic date, and I won't spend a lot of time discussing them, as all new rifles are "post ban" configs.
In general, a "preban" will have a normal flash hider, a bayo mount, and possibly a folding/collapsing stock.
Upper receiver: Many barrel types to choose from, ranging in length from 16 to 24 inches, varying weights, for general use, target, varminting, and so forth. Your basic choices are stainless steel or chrome moly for long distance/competition use, or chrome lined for ease of maintenance. You also can choose between a "flat top", which makes scope mounting easier, or the classic AR handle, which makes scope mounting somewhat problematic. Detachable handles are available for most flat tops. At the end of the muzzle, you can go with a naked crown, or one of various types of muzzle brakes.
Lower reciever: They're all pretty much the same, although you can select a 2 stage trigger rather than the military 1 stage trigger. Post ban lowers have a hump built into them which prevents the installation of the M-16 autosear, and thus pleases the ATF. A lot of people wind up replacing the GI spec grip handle sooner or later, for something else that fills the hand better.
Buttstock: Basic choices are psuedo collapsible, normal GI looking thing, or custom wireframey looking things.
Basic models, in a broad brushstrokes:
Rifles intended to approximate full length M-16's, in an AR sort of way.
Rifles intended to approximate M-4 carbines. These are typically 16 inch barrells with the "half length" handguards. The front sight post is closer than normal, as the gas block is build into it.
Dissipators: I've only seen these by Bushmaster. They're 16 inch carbines, with a full length handguard, a proprietary gas block, and the sight post is out front, at the correct sight distance.
Specialty: Floating handguards, varminters, carbon rifles, etc.
Hope that gives you the lay of the land.