"There's a special pleasure in shooting with primes. When you're restricted to one focal length, you can learn to see in it: visualize the picture even before putting the camera to your eye. I'm sure that with experience and talent this works for zooms too; I've just never gotten that far." -Petteri Sulonen

In still photography, a prime lens is a lens with a fixed focal length. A zoom lens, on the other hand, has a variable focal length. Zoom lenses, due to their greater versatility, have largely supplanted prime lenses. However, with the ever increasing affordability and thus popularity of DSLR photography, many of us who previously shot with fixed lens cameras now have the option of trying different types of interchangeable lenses. Amongst these, prime lenses offer a number of clear advantages.

1) They are lighter and more compact.
2) They often have simple designs, which are optically less compromising than zoom lenses. Modern zooms are very, very good. However, the greater the zoom range (ie, 4x), the more compromise is typically involved. For example, the wonderful Canon EF 24-105mm f/4L IS USM, which costs about $900, has a fair amount of barrel distortion at 24mm; whereas the EF 24mm f/2.8 prime lens, which costs about $200, has none.
3) More often than not, inexpensive primes offer better image quality than inexpensive zooms.
4) They are usually faster (greater maximum aperture) than zoom lenses of similar cost.

In addition to these quantifiable qualities, primes offer an additional advantage which is much less clear. As Petteri mentioned, there is a "special pleasure" in shooting them. With a prime, a given focal length takes on the charming character of familiarity. A prime can also encourage one to see in a certain way. For example, when I go out with the versatile 24-105mm zoom lens, which covers a focal range from ultra-wide to moderate telephoto, I have a tendency to stay further away from my subject and telephoto my way in. The effect is that I am less interactive, my perspective renders subjects more flat, and I end up including less environment in my candid photos of people. Each of these three factors has a negative influence on my shooting outcomes, and together their negative effects are additive if not synergistic. Now many zoom lens users would be quick to point out that not a single one of those effects is due to any inherent weakness of my zoom, and they would be correct. After all, if I mentally tape my zoom at 28mm, then it effectively becomes a 28mm f/4 prime, and not a bad one at that. However, my mind is weak, and I am guessing that I am not the only one. Furthermore, shooting with primes has definitely affected the way I use my zooms. These days, having learned from a couple of my prime lenses, I make better use of my feet, spend far less time on the telephoto end of my zooms, and take in more of the big picture. Even so, I suspect it will be many years before I progress to the point where prime lenses no longer push me to see better.

Amongst the more objective advantages of shooting with primes, the size and weight advantage is the most significant one from my standpoint. Today I visited the Baltimore Inner Harbor as well as Fells Point with my two boys, Oliver and Philip, aged four and two. For nearly half of my photos today, I was either carrying Philip in my left arm or holding his hand. There is simply no way I could have managed with a big heavy zoom like the 24-70L, and the smaller 24-105L would have been pushing it. Managing the 5D with a 28mm f/2.8 prime attached was a genuine pleasure. It would have been no problem to throw a 50mm prime in my jacket pocket had I wanted the security of being able to change focal lengths. However, I found that once I was in the 28mm mindset, I didn't even think of changing lenses.

Over in the DPR Canon Lens Forum, two common discussion threads are "Which one lens should I buy for my DSLR?" and "I'm leaving Canon for Nikon because Canon doesn't have an 18-200mm lens." The market dominance of zooms over primes has been firmly established, and I'd be the first to admit that prime lenses aren't for everyone. However, I would encourage any DSLR user to at least try taking pictures with a prime.

Right now, 28mm is my favorite focal length. If you are a zoom user, you might not have a favorite focal length. Maybe you think all the focal lengths are special, or perhaps none of them. However, if you give a couple inexpensive prime lenses a chance, I'll bet that before long you'll have a favorite focal length too.