Canon 16-35mm f4L IS 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 16 - 35mm. With a max aperture of f4 at 16mm, we can not consider this lens fast on wide end but f4 at 35mm is not too bad for tele end.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Canon 16-35mm f4L IS USM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body.