Stock images may be sold under three license types:
Editorial Only: these can only be used in narrative, factual, non-promotional works (i.e. newspapers, magazines). They are sold at a fixed price.
Rights-Managed: for these, the Buyer and Seller (photographer) negotiate a price based on the intended usage. The Buyer is granted a license only for the declared usage.
Royalty Free (standard): these can be used for any purpose, including advertising. A single fixed price grants rights to the Buyer, who can then use the image multiple times for any purpose.