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 35 mm film, 50 mm is a normal focal length. Typical telephoto lenses are 85 mm (28°50') and 100 mm (24°). They are used for portraits mostly. Other telephoto lenses that are used often are 135 mm (18°), 180 mm and 200 mm. Many are zoom lenses.
Longer focal lengths (300 to 1200 mm) are used in sports and nature photography.