For the last few years, my fine art photography has been represented by Corbis, which is one of the largest stock photography agencies in the world.
Basically, clients who are looking for a photo (for a brochure, a poster, greeting card, billboard, calendar...WHATEVER) search the enormous library of images that Corbis has, and then they buy the rights to use a photograph for a specific amount of time.
For example, one of my images was just purchased for a book cover, which was pretty exciting!!
Check it out...