Stock photography is basically a library of film photographs, illustrations or images that are available for public use. Of course there is a catch to this type of photography as you usually have to pay a fee to use a photograph or image, however if you search hard and long enough you may be able to find some for free. So it is essentially a photo for rent or a sale service as the images are sold for use over and over again or for permanent use.
A good photo may make somebody quite a bit of money. Most stock photography outlets have their images online for you to browse through. Stock photographs are available in all areas of photography from sports and action shots to landscape and animal photos. It's a good bet that somebody somewhere has the type of photo that you need.