Wide Angle
An excellent tool for capturing the jubilant bustle of a packed dance floor or the entire audience in front of the altar, the wide angle lens allows you to fit a lot of action into the frame. Focal lengths for wide angle lenses range from 17mm to 35mm, with 28mm being one of the more commonly used lengths. For an even wider field of view and a slight fisheye (distorted outward bulge) effect, you may try an extreme wide angle lens. Look for lenses with a wide maximum aperture (lens opening) such as f/2.8 or f/2.0 as this constitutes high performance even in dimly lit interior spaces.
50mm Prime Lens
The 50mm prime (fixed focal length, non-zoom) is a staple of most any photographer's kit because of its compactness and optical quality. It allows for extremely wide apertures such as f/1.2 and f/1.4, making it an excellent performer in nearly any lighting conditions. Because of its average focal length--not wide, but not telephoto--it is often treated as a general use lens, capturing everything from portraits of the bride to close-ups of the wedding cake's top tier. For guaranteed results no matter the lighting, try carrying one camera body affixed with a 50mm prime and another sporting a wide or telephoto lens.
Telephoto-Zooms
When you can't get up close to your subject, as during the vow-taking of the bride and groom, a trusty telephoto-zoom lens will help compensate. A common and reliable length for a telephoto is 70-200mm. This broad range of focal length makes it a perfect choice for portraiture and for certain shots which must be composed from a distance. Because of the number of different optical elements in telephoto-zooms maximum apertures tend to be a bit smaller (f/2.8, 3.5 and 4 are common), making them less competent low-light performers.
Auto-Focus
Although not necessary, lenses with an electronic automatic focus (AF) feature will make your job as a wedding photographer much easier. Amidst the constant movement and action at the wedding, maintaining sharp focus on your subject using a manual adjustment ring will be very difficult. To focus using an AF lens, simply compose your picture and depress the shutter button halfway. This locks focus on your subject until you fire the shutter. If your subject moves, release the button, recompose, and depress it again to achieve proper focus. A lens with auto-focus capability will be named accordingly, such as "AF Nikkor 50mm f/1.8D."