Recognisable by the extreme distortion that they provide, fisheye lenses are commonly seen in extreme sports photography.
This distorted effect is created by the lens being ultra-wide, meaning it crams a greater degree of detail into your frame. If you have ever looked through the peephole on a door, you are familiar with the fisheye effect.
To the adventurous, fisheye lenses can be used to add flair and creativity to a wide range of photographic styles, including landscape and portraiture. The trick is, learning how the effect is created and using it to your advantage.