By tele lenses we mean anything with a longer focal length than 50mm. They range from short teles of about 70mm to very long ones of more than 500mm, which are used for capturing action at major sporting events or wildlife shots.
With a telephoto lens you can crop in tightly on the subject, to capture detail or isolate it from its surroundings which makes for compositions with lots of impact.
A further advantage of a telelens is that, since it has limited depth of field, it keeps the background out of focus. The longer the focal length of the lens and the wider the aperture, the less depth of field there is. Thus it follows that focusing with a tele is much more critical as there is not much room for error. A long telephoto at maximum aperture will only give a few inches depth of field for a subject five meters away.
As opposed to wide-angles which open up perspective, tele lenses compress space, making subjects in a scene appear closer together than they actually are.
When using a telephoto it is especially important to hold it steady, as the effect of camera-shake will be magnified as the lens gets longer. A useful guide would be to use a shutter speed equal to the focal length of the lens or one stop faster to be on the safe side.