Stock footage is a film or video footage used in a production for which it was not at first filmed. This can be clips from motion pictures, interviews, news or any footage. Motion pictures often use stock footage to reference other films or periods in time. Documentaries often use stock footage for this reason. In order to more accurately depict the time or subject discussed in the documentary, they will use footage or news segments from the period dealing with the subject matter.
Stock footage can be used to reduce costs. This is often the case in television shows, as it can be expensive to constantly film certain common shots. For example of this is the known popular television show, “Moesha.” In order to combine scenes and make the show run more smoothly for viewers, producers of the show inserted images of exteriors and interiors which had been recorded during previous episodes. The most seen were the images of the outside of the apartment or the coffee shop, Central Perk.
One of the largest producers and distributors of stock footage is the United States government. Much of the stock footage with historical significance is available to the public and can be used by producers free of charge. In other instances, such as using footage from another motion picture or a newscast, producers must purchase the rights to each individual clip from the archive in which they are stored.(http://www.steves-digicams.com/knowledge-center/how-tos/film-and-video-production/what-is-the-difference-between-basic-stock-footage-and-archive-footage.html#b)