Canon 100-400mm f4.5-5.6L IS USM 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 f4.5 at 100mm, and f5.6 at 400mm, the lens is quite slow at both ends.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Canon 100-400mm f4.5-5.6L IS USM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body. Canon 100-400mm f4.5-5.6L IS USM was replaced with Canon EF 100-400mm f4.5-5.6L IS II USM. Follow the link to read our comparison of these cameras: Compare Canon EF 100-400mm f4.5-5.6L IS USM vs Canon EF 100-400mm f4.5-5.6L IS II USM