Zoom is a classification of lenses in the field of photography, or cinematography, or what-have-you. Zoom lenses have a variable focal length, which simply means that they can make the subject matter appear closer or farther away without actually having to move your feet. What actually happens is that you change the angle of vision and the magnification.

Here are some examples of zoom lenses used with a 35mm camera. The focal lengths to get the same area of coverage would change if you are using a medium format or large format camera. A 17-35mm zoom lens would be considered a wideangle zoom and would be great for scenic landscapes or photographing a large group of people. Conversely, a 200-400mm zoom lens would be used to take pictures of sports or animals or other things that are difficult to get close to. This would be a telephoto lens. A 35-70mm zoom lens would fall in the normal focal length, and would be good for everyday shooting.

Zooms are a relatively recent advancement in the field of photography. They have been around for quite some time, actually, but only in recent years has the optical quality become good enough for them to be considered indespensable tools by professional photographers. The nice thing about zoom lenses is that you can "get it all in one package". One zoom lens could do the job of 2,3,4 or more fixed-focal length lenses. This will save considerable weight and cost. Some of the drawbacks are slight loss of optical quality and generally smaller apertures AFAIK.