Most lenses sold these days are zoom lenses-- they zoom in and out so that you can compose your shot without even moving your feet. They are convenient (especially when you can't move closer to a subject because of a barrier, distance or perhaps it's a bear that you don't want want to get too cozy with). Prime lenses are for only one focal distance. The advantage of prime lenses are that the optics are designed exactly for that focal distance, while a zoom lens needs to have the "flexibility" for a wide range of focal distances but with a trade off of being less sharpness, especially at the extremes. The prime lens will give you a sharper picture, but you have to use your feet to zoom.
This photo was shot with my new prime lens, a 50 mm f/1.8 D. The f/1.8 designates that is also a fast lens and that the f stop goes all the way down to 1.8. (f stop means how much the shutter opens to let light in). I was able to shoot this photo without a flash at a shutter speed of 1/124 sec with just ambient lighting (bluish natural light coming in from the window and a halogen light on the ceiling providing the warmer yellow light).