Photographs that are rented for the purpose of editorial illustration or advertisement.
Publishers access stock either through CD bundles (Corel will sell you a little grey suitcase with 100 PhotoCDs chock full of pictures, organized by subjects: Native American, Birds, France, Airplanes ...) or through an agency.
An agency will root in its vast files (we are talking about millions of slides here) and rent (more precisely license) an image for a certain purpose.
The price always varies with the type of publication and with the estimated circulation.
Using photo stock is usually cheaper than setting up a photo shoot. It leads to a certain uniformity of vision, though. Of course, for some applications using stock is the only viable option: if you want to illustrate an article on water scarcity on the Cowlick Illustrated Chronicle, you don't have the money to dispatch one photographer to Namibia, one to Mexico City, one to Mongolia etc.
Photo stock catalogs positively brim with images of pensive businessmen, happy couples (in various colors and races and sexes), jocks, cute babies, happy families and generic poses like "reading a book", "meditating", "looking at a computer screen".