well, wikipedia and pretty much everywhere will tell you postrock is 'rock instrumentation used for nonrock purposes', how utterly drawl. so instead, I will provide you myspace links dabbling in what it actually
sounds like:
Explosions in the Sky
Mogwai
Godspeed You! Black Emperor
Sigur Ros
those are some of the big influencial acts in the genre. Essentially it's rock music that's pretty and moving.
and these links also for all you little kiddies who want to impress your 14 year old friends
edit: some classic is fucking good too. mainly thinking neoclassical here, but Max Richter instantly spring to mind. then Needs More Sources. and that's all that instantly springs to mind actually