Prime lenses (sometimes called fixed lenses) result in a sharper focus than zooms simply because of the way they are constructed (I'll spare you the details). Depending on the lens it is sometimes possible to get very close to the subject and also to shoot in extremely low light situations.
I took this photo while at the pool this afternoon. It is closely cropped (love those 12 MP). All of those dots in the water droplets are reflections of the sun. To see the full photo go to the Shooting Blog.
$.34 (lousy day on the streets two days ago)