2-channel is left and right speakers, yes. It may or may not be stereo, could just be mono on both speakers - depends on the source.
5.1 refers to 5 main speakers and a subwoofer, driven individually, to provide a "surround sound" experience. For instance, you could hear an aeroplane fly from behind you, overhead, then off to the front-right as it flies on screen.
The 5 main speakers are front-left, front-right, rear-left, rear-right and a mono centre speaker mainly for vocals. The subwoofer (the .1) just handles very low notes (nice, deep rumbles to shake the room)