Sigma 18-50mm F2.8-4.5 DC OS HSM is an Auto Focus (AF) SuperZoom lens with APS-C / DX max sensor coverage. When mounted on a APS-C / DX sensor camera, it will give you an FF equivalent focal range of 27 - 75mm. With a max aperture of f2.8, this lens is fast on wide end, but only moderately fast at the tele end with max aperture of f4.5.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Sigma 18-50mm F2.8-4.5 DC OS HSM is not a weather sealed lens, so you have to be extra careful against rain and dust.