The first movie trailer came from the Loews Cinemas company in 1913 for a film called "The Pleasure Seekers." It was a short promotional that played in Loews theaters before the film. The practice was adopted and used in all of the Loews theaters and later caught on throughout the industry. The early trailers were made of key scenes from the film cut with text. The company that created these trailers through the 1950s was the National Screen Service.
Starting in the early 1960s, directors became more involved in the trailers, leading to more stylistic versions, such as montages of scenes, silent or sparse music as well as the thrilling, over-the-top promos we are now used to and the newer trend of the "teaser trailer" which is often released a year before the film is set to be in theaters and often only provides a teasing glimpse, image or logo of the film.
The purpose of a film trailer is basically to make you want to see the movie, and there are a number of techniques that are employed in order to tantalize the viewer. They have to be executed within two and a half minutes, which is the limit imposed by cinemas.
A film trailer is a marvel in commercial editing. It relies on the use of scenes from the film, cut together to suggest the plot and the genre and to showcase the stars of the film. Music is also the major component to a trailer, whether it be a string-laden orchestral piece to highlight the drama of the story or sparse piano and silence like the trailer for Stanley Kubrick's film "Eyes Wide Shut." The music communicates emotion, in shorthand, because the trailer cannot give enough depth in its short amount of time. There is also sometimes a voice-over, which gives information in a very sensational tone, to entice the audience and fill them in on the emotional content.
The parts of the trailer as eluded to earlier, are the music and images from the film and sometimes the voice-over (though this isn't as widely used at this time) but also there are a few other pieces that are necessary to the trailer. First, there is the MPAA rating, which is either G, PG, PG-13, R or NC-17. This informs the viewer of things they may find objectionable and also gives a bit of insight about what kind of film it will be. Very often, the director, the writer and the actors are listed or introduced in a voice-over.
All these parts are assembled into a film trailer, to entice the viewer into wanting to watch the movie, whether it be for the plot, the talent involved or its rating, But most often, it's because of all these things combined.