The lens is probably the most important part of the camera. Photography is all about capturing light, and all the light you capture passes through the lens. On less expensive cameras, the lens is built into the camera's body. Single lens reflex (SLR) cameras have interchangeable lenses. The camera body has a metal ring where the lens attaches. The attachment point also has electronic connection points so the camera can control the focus and zoom motors on the lens.
An important factor in choosing a lens is focus length. This generally refers to the length of the lens itself, and is measured in millimeters. A 50 mm lens is considered average, good for shooting subjects that are 33 to 65 feet (10 to 20 meters) from the camera. Wide angle lenses have shorter focal lengths that can capture large panoramic scenes or large subjects that are relatively close to you. Telephoto lenses, which can have focal lengths of hundreds of millimeters, are basically telescopes mounted to a camera. They make far away subjects seem closer, but with a limited field of view.
If your camera has a zoom lens, then the focal length changes as you zoom in and out. Remember to never use digital zoom -- the camera is simply enlarging the pixels to make the subject seem closer, which reduces image quality.