Theres two types. Classic and RT. Classics usually (not always) have a dovetail front to accept a dovetail foregrip or accessory. Classics also have classic sears. The difference between the classic sear and rt pro sear is that a classic sear has a different trigger pin length, and pivots on a pin instead of a screw in axle.
The RT like I said has a screw in axle (and the rail must have the threads for the axle to fit), and usually has two holes for the 10-32 threaded screws that hold the foregrip..
Rogue's rails (
www.TheMagSmith.com) have adjustable foregrip slots. Instead of just two holes, they have a slot so you can move the trigger where you like. His also have a rear accessory slot, and you can rig up somethign custom there, or add one of his cool RPG Gripper snatch grips.
The rear slot is also adjustable.
Rogue's rails are THE best, hands down. Only ones to buy.