What is a fisheye lens?
Most people think that any old lens that has a bulbous front element is a fisheye lens. That actually is incorrect.
Lenses such as the Nikon 14-24mm f/2.8G and the Tokina 16-28mm f/2.8 both have bulbous front elements, however, neither of these lenses are fisheye lenses.
Fisheye lenses are lenses that are INTENTIONALLY distorted. Lenses are classified as fisheye if their field of view is 180 degrees. This means that you should be able to see the nose of the person standing right next to you if you were using a fisheye lens.
So, in theory, even if the focal length of a fisheye lens might be greater than another wide angle lens, the fisheye lens might actually get more into the frame with its wider field of view.
In the above image, I practically had my lens glued to the window. But notice the distortion and how much of the sides I can actually see. Definitely a fisheye lens.
This was taken with a d90 along with a Rokinon 8mm f/3.5 Fisheye lens at around f/8, I believe. I was trying to achieve a panning effect.