There is a plethora of lenses out there for the budding photographer. You'll need to choose one that's suitable for the type of photography you practice, but when it comes to specifics, here are a few pointers on what lens is best for what scenario:
On full-frame DSLRs, standard lenses fall between 40mm and 55mm, though 50mm is the accepted norm. Closest to the field of view of the human eye, standard lenses offer an undistorted perspective and are often used for flattering portraits. The closest equivalent for APS-C sized sensors is a 35mm lens. Most DSLRs come with a standard zoom, which spans from moderate wideangle to short telephoto. Consumer lenses tend to have a lower maximum aperture.
With shorter focal lengths and wider angles of view than standard lenses, wideangle lenses are employed by landscape and reportage specialists. Remember you'll need a shorter focal length on many DSLRs to get the equivalent field of view if you don't have a full-frame sensor. There's a plethora of wideangle lenses available, from 8mm ‘fisheye' lenses to 28mm lenses. Wide zooms are increasingly popular and effective.
If you've ever wondered how photographers fill the frame with small subjects such as petals and insects, the answer is the humble macro lens. Allowing for 1:1 (life-size) reproduction and focusing from as close as two inches, true macro lenses are specifically constructed for close-up photography. They are commonly available in focal lengths between 50mm and 180mm. The macro lens is not restricted to purely close-up photography - many photographers employ their services for portraiture as well.
Any lens that weighs in with a focal length in excess of 50mm is said to be a telephoto lens. Traditionally, short telephotos (between 70mm and 120mm) are ideal for portrait photography, but that 50mm standard lens you used on your old film camera offers the equivalent angle of view to a 75mm lens (in 35mm terms), so is now an ideal lens for portraiture. Longer focal lengths (between 135mm and 300mm and above) are perfect for sports and wildlife photography.