Canon 16-35mm f2.8L 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 f2.8, this lens is fast on wide end and equally fast at 35mm with a max aperture of f2.8.
Canon 16-35mm f2.8L USM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body. Canon 16-35mm f2.8L USM was replaced with Canon EF 16-35mm f2.8L II USM. Follow the link to read our comparison of these cameras: Compare Canon EF 16-35mm f2.8L USM vs Canon EF 16-35mm f2.8L II USM