A zoom lens is meant to take pictures at a variety of distances while telephoto lenses are meant for a single distance. Telephoto lenses are usually defined as any lens more than 50 millimeters, though others define it as any lens more than 100 millimeters.
Zoom lenses are convenient because they can take the place of many other lenses; for that reason, they are favored by amateurs, since zoom lenses are more imprecise than other lens types. Some zoom lenses can range from 18 millimeters up to 200 millimeters. Sometimes, telephoto lenses between 50 and 100 millimeters are defined as "portrait" lenses instead.