Totally not stolen from Wikipedia:
The first trailer shown in an American film theater was in November 1913, when Nils Granlund, the advertising manager for the Marcus Loew theater chain, produced a short promotional film for the musical The Pleasure Seekers, opening at the Winter Garden Theatre on Broadway. As reported in a wire service story carried by the Lincoln, Nebraska Daily Star, the practice which Loew adopted was described as "an entirely new and unique stunt", and that "moving pictures of the rehearsals and other incidents connected with the production will be sent out in advance of the show, to be presented to the Loew’s picture houses and will take the place of much of the bill board advertising". Granlund was also first to introduce trailer material for an upcoming motion picture, using a slide technique to promote an upcoming film featuring Charlie Chaplin at Loew's Seventh Avenue Theatre in Harlem in 1914.
Trailers were initially shown after, or "trailing", the feature film, and this led to their being called "trailers". The practice was found to be somewhat ineffective, often ignored by audiences who left immediately after the feature. Later, exhibitors changed their practice so that trailers were only one part of the film program, which included cartoon shorts, newsreels, and serial adventure episodes. Today, more elaborate trailers and commercial advertisements have largely replaced other forms of pre-feature entertainment, and in major multiplex chains, about the first 20 minutes after the posted showtime is devoted to trailers.
Up until the late 1950s, trailers were mostly created by National Screen Service and consisted of various key scenes from the film being advertised, often augmented with large, descriptive text describing the story, and an underscore generally pulled from studio music libraries. Most trailers had some form of narration, and those that did featured stentorian voices.
In the early 1960s, the face of motion picture trailers changed. Textless, montage trailers and quick-editing became popular, largely due to the arrival of the "new Hollywood" and techniques that were becoming increasingly popular in television. Among the trend setters were Stanley Kubrick with his montage trailers for Lolita (1962), Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb (1964), and 2001: A Space Odyssey (1968). Kubrick's main inspiration for the Dr. Strangelove trailer was the short film Very Nice, Very Nice (1961) by Canadian film visionary Arthur Lipsett. Pablo Ferro, who pioneered the techniques Kubrick required as necessary elements for the success of his campaign, created the Dr. Strangelove trailer, as well as the award-winning trailer for A Clockwork Orange (1971).
Many home videos contain trailers for other movies produced by the same company scheduled to be available shortly after the legal release of the video, so as not to spend money advertising the videos on TV. Most VHS tapes would play them at the beginning of the tape, but some VHS tapes contained previews at the end of the film or at both ends of the tape. VHS tapes that contained trailers at the end usually reminded the viewer to "Stay tuned after the feature for more previews." With DVDs and Blu-rays, trailers can operate as a bonus feature instead of having to watch through the trailers before the film
Ah, how far we have come. Here are some great examples of this unique art for. True avant-garde material!