So what is a prime? A prime is a lens that only has a single focal length, or put more simply, a prime is a lens that doesn't zoom.
Zoom lenses are common, often inexpensive, and a few of them are pretty amazing optically speaking. Why would you ever want to limit yourself to a single focal length?
- Speed. Generally speaking, for full frame cameras the very best zoom lenses top out at around f/2.8. Prime lenses on the other hand can be commonly found at f/1.4 or f/2.0. Some lenses, like Leica's Noctilux, have been known to be as fast as f/.95. Obviously this has a dramatic effect on both the amount of light gathered, and depth of field.
- Image quality. I need to stress that this isn't always true but typically primes are sharper and have less distortion than equivalent zooms. But, as is the case with most things, some lenses are works of genius and others were clearly designed on the Friday before a big holiday. The fact remains that the fewer things a lens needs to do the fewer engineering compromises have to be made.
- Price. I'm going to address this because this is a very common reason people are given for choosing prime lenses. The statement that primes are cheaper is only somewhat true. Lens for lens, it's certainly true - primes are usually cheaper. However once you factor in the need for multiple lenses to cover what a single zoom can, it usually turns out that zoom lenses are cheaper.
- Weight. Provided you can precisely say what you'll need before you leave, primes are much lighter. "I'm going to the theme park and I'm bringing a 28mm lens. It'll be perfect because I can take environmental portraits of the kids and that's all I want." As soon as you have to bring the whole camera bag because you don't know what you'll want, then prime lenses are both heavier and less convenient.
- Special features. Zoom lenses try to do everything acceptably well. Primes on the other hand often have a primary purpose... no pun intended. Some primes are made for portraiture and can modify the bokeh(out of focus area) with the twist of a ring; others are built for architecture and can tilt and shift to correct for converging lines; still others are built for macro photography and can get so close that you can take a headshot of a spider. The list goes on!
- Discipline. It is very easy to just zoom in to get the framing you want. You zoom in, only paying attention to your primary subject and ignoring what it's doing to your background, or the distortion or compression related to your focal length. It's easy to not know what focal length you're at, which makes it hard to replicate what you've done. Primes force you to think before you change focal lengths. With primes you can get used to a certain lens doing a certain thing which improves your consistency. It's entirely possible to do the same thing with a zoom by deciding to slow down and pay attention when you change your focal length, but most people don't.
So those are primes in a nutshell. Enjoy shooting!