Telephoto lenses have a narrower field of view than wider optics, enabling you to crop in on details and magnify more distant subjects so that they appear closer to you than they actually are. They're perfect for photographing things that you can't get physically close to (at least, not safely) such as elusive wildlife subjects and many sports. They're also used for selective in-camera cropping, so you can fill the frame with a single tree in a landscape, or person in a crowd. Telephoto lenses imbue your images with certain aesthetic traits, such as shallower depth of field, and the sense of compressing distance so that elements further away in your scene seem right on top of closer ones.
Although there's a wide choice of prime telephoto lenses, zooms offer the huge advantage of allowing you to zoom in or out to get the exact framing you want, which is especially useful given that in many of the situations in which you'd use one you may not be able to freely move around.
There's a huge variation in zoom ranges on offer. Some start at 50mm but the most common range goes from about 70mm up to 200mm or 300mm.
Telephoto lenses enable you to zoom in on subjects from further away - perfect when you can't get close, or when moving closer would frighten your subject. They have the effect of compressing the sense of distance, and producing shallow depth of field. This lizard (shown here) was photographed using the 400mm end of a Canon 100-400mm f/4.5-5.6 L IS USM, fitted to an EOS 40D. Most sports photographers routinely use telephoto lenses for shots like this horse race.
Lens Top Tip
The very best quality tele-zooms are invariably the 70-200mm ones with f/2.8 maximum apertures. These offer the ultimate optical performance across the frame, albeit at the expense of greater cost and physical bulk.