Before we get into the different kinds of lenses, it pays to know a bit about what makes them so different. The most basic measurement that differs between lenses is focal length, represented in millimeters and often in a range, like “50-200mm.”
The focal length is essentially the zoom level of the lens (you can see how a different focal length affects images with one of these online DSLR simulators). A 300mm lens will make far-away subjects appear a lot closer than a 24mm lens. A lens with a range of focal lengths (e.g. 18-55mm) is a zoom lens, and these are frequently bundled with entry-level cameras.