Canon 17-40mm f4.0L USM 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 17 - 40mm. With a max aperture of f4 at 17mm, we can not consider this lens fast on wide end but f4 at 40mm is not too bad for tele end.
Canon 17-40mm f4.0L USM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body.