fisheye lenses are special lenses which are designed to give a 180deg field of view and extreme barrel distortion is a characteristic of these lenses
there are 2 main image types:
diagonal fisheye lenses - these images fill the full rectangular frame of your image
circular fisheye lenses - these lenses give a circular image thus the area of your frame outside of this circle will appear as black
fisheye lenses are very much a niche lens which is primarily used for creative photography
they are particularly useful for creating visually stimulating images, artistic or humourous imagery
their short focal length combined with an aperture of f/8-11 means that most of the image will be acceptably sharp, and it may be possible to have subjects almost touching the lens to be sharp while the background gives a relatively sharp and very wide perspective.
their protruberant convex front lenses mean that filters cannot be mounted
some lenses allow small filters to be inserted into a rear slot
given the niche nature of the lens, when considering purchasing one, it may well be worth considering a reasonably affordable one with excellent optics in a compact, light size so you will actually take it with you in your photographic adventures