Your favorite "kit zoom," only better
Your DSLR might have come with a short-range zoom lens. In a great many cases, this is what we call a "kit zoom" lens, and it does a great job of delivering a lot of versatility while not adding a lot of overall cost to your camera purchase.
The thing that might not be immediately obvious is that they often trade light sensitivity, particularly as they're zoomed in, for those cost savings. So what if you had the great versatility of a kit zoom's focal length, matched with a light-gathering wide aperture that stays constant through the zoom range? You'd have this in-demand camera-bag staple, the Sigma Photo 17-50mm f/2.8 zoom lens.
Designed to give you everything from a panorama of a sun-dappled river winding through Big Sky country, to an intimate portrait of your spouse squinting as he surveys that landscape's majesty, this lens is ready for most day-to-day opportunities, whether they're in daylight or low light.