Sigma 100-400mm F5-6.3 DG OS HSM is an Auto Focus (AF) Telephoto Zoom 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 100 - 400mm. With the max apertures of only f5 at 100mm, and f6.3 at 400mm, the lens is quite slow at both ends.
Sigma 100-400mm F5-6.3 DG OS HSM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body.