What is STOCK PHOTOGRAPHY?
Stock photography is a collection of photos created by one or many photographers, which are used by designers, illustrators, those in advertising, etc. The photos are marketed by a Stock Photo Agency and people pay a fee to license the photos for a specific use. Essentially, they are "renting" the photo.
They use these photos because often, it is difficult, expensive or impossible for them to shoot or obtain the photo themselves. For example, it would be quite costly and time consuming for a NYC based designer to fly to the Amazon just to shoot the one photo he needs for an advertisement. A stock photographer who has already been to that location may have a wide selection of great photos available!
Stock photography isn't limited to exotic subjects and locations, though. Even everyday people, objects, and locations make useful stock photos. Suppose that same designer needs a shot of someone with a great smile for a toothpaste ad... Why should he hire a photographer, a model and rent a studio when there are already many great photos out there that he can use? He shouldn't - stock photography is the answer!