A zoom lens has a variable focal length. By turning the zoom ring, you move optical elements inside the lens to achieve a different angle of view. This means that you can make objects appear larger by turning the zoom ring in one direction, or fit more objects into the frame by turning it in the opposite direction.
Zoom lenses have two specifications which represent the two extremes of the zoom range, for example – 70-200mm. Such a lens may act as a 70mm focal length lens, a 200mm focal length lens and everything in-between. In addition, zoom lenses can also have variable aperture ranges. On many consumer zoom lenses, you will often see something like f/3.5-5.6, which represents the maximum aperture of the lens at different focal lengths. For example, a lens like Nikon 18-55mm f/3.5-5.6 will have a maximum aperture of f/3.5 at the shortest focal length of 18mm, while at the longest range of 55mm, its aperture will be limited to f/5.6. On the other hand, most professional-level zoom lenses will have a single maximum aperture throughout the zoom range. Examples of zoom lenses are: Nikon 18-200mm f/3.5-5.6G VR II and Canon 16-35mm f/2.8L II.