Canon 600mm f4.0L IS USM is an Auto Focus (AF) Telephoto Prime lens with 35mm FF max sensor coverage. When mounted on a 35mm FF sensor camera, it will give you an FF equivalent focal range of 600mm. With a max aperture of f4 at 600mm, this is a moderately fast lens.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Canon 600mm f4.0L IS USM is not a weather sealed lens, so you have to be extra careful against rain and dust. Canon 600mm f4.0L IS USM was replaced with Canon EF 600mm f4.0L IS II USM. Follow the link to read our comparison of these cameras:
Compare Canon EF 600mm f4.0L IS USM vs Canon EF 600mm f4.0L IS II USM