[kallisti]

what's a picatinny rail?

[Long Cut]

That's the notched-looking rail on the carrying handle and on the forearm. It permits easy, fast attachement of scopes, lights, and other accessories without change of zero. Most newer AR's have them. They are available for attachement to older ones, as well.

[RKV]

The parts of the rifle stock that can mount other goodies (e.g. flashlights). Look at the edges and see the grooves used to attach the add-ons.

[NYFriend]

Universal (sort of) mount for optics, etc.

[Gothmog]

It better not be a pickaninny rail, that would make it a racist gun.