Sigma 12-24mm F4.5-5.6 EX 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 the max apertures of only f4.5 at 12mm, and f5.6 at 24mm, the lens is quite slow at both ends.
Sigma 12-24mm F4.5-5.6 EX DG HSM is not a weather sealed lens, so you have to be extra careful against rain and dust. Sigma 12-24mm F4.5-5.6 EX DG HSM was replaced with Sigma 12-24mm F4.5-5.6 II DG HSM Canon EF. Follow the link to read our comparison of these cameras:
Compare Sigma 12-24mm F4.5-5.6 EX DG Aspherical HSM Canon EF vs Sigma 12-24mm F4.5-5.6 II DG HSM Canon EF