"Fisheye lens is a wide-angle lens that takes in a broad, panoramic and hemispherical image." [Wikipedia]The above quote is taken from the Wikipedia page for Fisheye lenses. Fisheye lenses are used more in certain genres than others, for example "Rap" and "R 'n' B".
There are two types of Fisheye Lens; Circular and Full Frame.
Here is a quote regarding Circular Fisheye lenses:
"The first types of fisheye lenses to be developed were "circular fisheyes" — lenses which took in a 180° hemisphere and projected this as a circle within the film frame."And here is a quote for Full Frame Fisheye lenses:
"The picture angle produced by these lenses only measures 180 degrees when measured from corner to corner: these have a 180° diagonal angle of view, while the horizontal and vertical angles of view will be smaller; for an equisolid angle-type 15 mm full-frame fisheye, the horizontal FOV will be 147°, and the vertical FOV will be 94°."Both of these quotes were taken from the following page of Wikipedia.