Sigma 12-24mm F4 DG HSM is an Auto Focus (AF) Wideangle 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 12 - 24mm. With a max aperture of f4 at 12mm, we can not consider this lens fast on wide end but f4 at 24mm is not too bad for tele end.
Sigma 12-24mm F4 DG HSM is not a weather sealed lens, so you have to be extra careful against rain and dust.