Billy,
The differences can be classified into 3 basic groups.
1. Rails that bolt onto your existing handguards. Cheap and not very stable.
2. Replacement handguards with the rails incorporated into them. These are the Fobus, Yankee Hill Mfg, and Surefire systems. I'm sure there are others as well. Materials may be plastic or aluminum. These are great to use to attach lights, vertical foregrips, etc... Attaching sights to these can be iffy due to the HG shifting. I know the surefire m73 system is designed to lock on and not move---a good platform for sights.
3. Free floating HG systems. These replace the HG, barrel nut, and HG retaining ring. These systems usually require the removal of the front sight base and any muzzel device to install. They are stable ( a good platform for mounting sights) and don't place any strain on the barrel when you rest them, or sling up, or whatever. These tend to be the most expensive, but you can find good systems for under $100 if you look (YHM, FRISH, etc...)
Hope that helps, feel free to ask if you need more info.
--usp_fan