Isn't there a sticky about shaders on this forum? Very briefly, a shader is something that does work in a video card. We used to have seperate shaders/work units until DX10. (DX10 doesn't require you to use unified shaders, but it makes much more sense to do that.)
BTW, keep in mind that because AMD and Nvidia went different routes with their shaders, you can't just say that AMDs 320 shader card must be more powerful then Nvidia's 128 shader card. The amount of work each companies shader can do varies, and they aren't 100% comparable.