Both fish-eye lenses and wide-angles cover a large area -- although fish-eye lenses aren't corrected for barrel distortions. i.e. straight lines that don't pass through the optical centre of the lens become distorted and that's why they tend to have that "curve" to them. You can get full-frame fish-eye lenses that provide a large enough area so that you don't see the circle within the frame (thus, the image is rectangular, as opposed to circular). You can even get 360 degree fisheyes.. but that's basically the jist of it: wideangles are corrected for barrel distortion, fisheyes are not.