Sigma 800mm F5.6 EX DG HSM 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 800mm. With the max aperture of only f5.6 at 800mm, it is considered a slow lens.
Sigma 800mm F5.6 EX DG HSM is not a weather sealed lens, so you have to be extra careful against rain and dust.