It is called a trailer as they used to be shown at the end of a film screening, but soon became placed before as viewers would leave after the film and not watch.
They are vital pieces of marketing for feature films, and are usually shown at the beginning of a film of a similar genre film. They are also shown on DVDs and Blu-rays, previously videos etc. They are used to attract attention to target markets on similar films.
Usually they are short and contain key conventions of the genre, such as fast paced, quick cuts and loud music for action films, etc. This amplifies the tone of the film.
The first promotional trailer was for a musical, showing the exerps of the production and raised awareness. Up until the late 1950s, film trailers were made by the National Screening Service which was contracted by film companies to create trailers for upcoming films. This lasted until the 80s.
Early trailers included short clips from the film alongside narration, a score soundtrack and screens with credits and cast run.
An example: Casablanca http://youtu.be/EJvlGh_FgcI
1964 Night of the Iguana - Andrew J. Kuhen http://youtu.be/HPdFDfQyi_c
He was an influential trailer producer who changed the way trailers were made. The company worked for 30 years creating some of the most influential trailers for huge films and film makers of the time, including Spielburg.
1894 - Thomas Edison develops motion picture camera and projector
1897 - Hand etched slides used for on screen advertising
1904 - Commercially produced advertising slides made by Lubin Co.
1912 - First text only trailer - "What Happened to Mary"
1915 - Development of scene trailer
1916 - Paramount announces official trailer policy
1919 - National Screen Service opens in. New York
1920s saw the NSS dominate trailer production and distribution
1933 - Invention of Optical Printer
1940s - 'Golden Age' of trailers
1946 - Post WW2 ticket sales decrease
1950s NSS systems begin to become exhausted
1955 - Saul Bass develops key art concept
1963 - Pablo Ferro creates first contemporary trailer "Dr. Strangelove"
1964 - Andrew Kuhen's "Night of the Iguana" reinvents trailer making
1972 - Birth of blockbusters and TV advertising expands
1975 - "Jaws" success highlights importance of trailers
1988 - Non-linear digital editing era begins
1994 - Internet era begins and video games become large place to advertise trailers
1998 - Release of online bad mobile trailers change styles of editing, appearance, emphasis
2000 - Consumption of trailers shift from theatrical, singular and collective experience to personal, repeated and home experience
2004 - New 3D era slows pace of trailer editing style
2011 - No need for DVD release date trailer due to films being released simultaneously theatrically and to home audiences
