Technically, telephoto lenses have a focal length or 60mm or more for a standard 35mm or full sensor size camera, though most people think of telephoto lenses having a focal length of 85mm or more. Lenses with a focal length greater than 300mm are considered to be super-telephoto lenses.
Telephoto lenses help us bring distant subjects closer, fill our frame with the subject, show detail difficult to see, capture action, and produce a shallow depth of field.