Most lens designs of any quality attempt to ensure that when you take a photo of a straight line, it comes out straight in the image. A few - mostly expensive single focal length lenses - achieve the goal of having virtually no distortion. It's rare to see this characteristic in wide angle lenses, but some (like my cherished Olympus 7-14mm zoom) even manage to keep distortion to a minimum at ultra-wide angles of view. At the other end of the scale is a class of lens that revels in not playing by the rules - the fisheye.
Fisheye lenses are curious things. They achieve a spectacularly wide 180 degree field of view by not even trying to keep things straight. The resulting distortion can vary from moderate to extreme, depending on the placement of objects within the frame, their distance from the camera and whether or not the camera is held level. The effect can be stunning, comical or downright disturbing. They're not the sort of thing you'd want to take all your pictures with, but used with care, and on the right subjects, they can produce arresting images.
I've never been much of a fan of fisheyes. I've seen too many photos where the rationale for using the lens appeared to be that the photographer had one. I also thought that there was too much cross-over with my 7-14mm ultra-wide. But having recently had the chance to play with one, I can see they provide an interesting perspective and can be a lot of fun. Perhaps I should fish around for one.