What is a prime lens? Well, it's a lens that isn't a zoom. A prime lens has a fixed focal length which means it has only one focal length. Examples of NIKKOR prime lenses are the AF-S NIKKOR 35mm f/1.4G and AF-S NIKKOR 200mm f/2G ED VR II, and the new AF-S NIKKOR 50mm f/1.8G. Some prime lenses are designed for specific uses, such as the Micro-NIKKOR (AF-S Micro-NIKKOR 85mm f/3.5G ED VR), Perspective Control (PC-E Micro NIKKOR 45mm f/2.8D), and Fisheye (AF DX Fisheye-NIKKOR 10.5mm f/2.8D ED) lenses.
If a prime lens gives you only one focal length, but a zoom gives you a range of focal lengths, what are the benefits to using a prime lens?
Prime lenses, are smaller, lighter and more compact than zoom lenses and tend to be faster, offering wide apertures of f/2.8, f/1.8 or even f/1.4. The large apertures let in more light, which benefits the photographer in a number of ways. First, you can use fast shutter speeds with large apertures to freeze motion. This becomes important when you're shooting in low light. By using a high ISO, and wide aperture, you'll be able to use high enough shutter speeds so you can hand-hold the camera.