Post production is the general term for the last stage of film production in which photographed scenes (also called footage) are put together into a complete film.
Post production is in fact many different processes grouped under one name. These typically include:
Typically, the post production phase of creating a film takes longer than the actual shooting of the film, and can take several months to complete.
Other film production stages include (very broadly) - financing, writing the screenplay, rewriting the screenplay (repeat), and the actual shooting.
Major post-production companies