In photography, a telephoto lens is a lens. There are two different uses of the word. The less specific use is to mean a lens that has a longer focal length than a normal lens. Their angle of view is smaller than that of a normal lens. With telephoto lenses things look closer than they really are.
The other meaning of telephoto lens applies to lenses that are constructed in a special way so they can be shorter than their focal length.
For 35mm film, 50mm is a normal focal length. Typical telephoto lenses are 85mm (28°50') and 100m (24°). They are used for portraits mostly. Other telephoto lenses that are used often are 135mm (18°), 180mm and 200mm.
Longer focal lengths (300 to 1200mm) are used in sports photography.
Different focal lengths [change]
Effect of different focal lengths on photographs taken from the same place: