Trailers, known to the general public as Coming Attractions or Previews, is the industry term for the commercials that commonly begin a feature film in a theater. They were named so, because they used to "trail" a feature. These days, trailers can be seen in theaters, on television, on the internet, and on videos or DVDs.
These days a typical trailer for a main Hollywood feature can cost half a million dollars. This is an important investment, because commonly you already have your audience gathered. They are in a theater. They are moviegoers. Other common ideas are to include a trailer of a movie that shares genres with the feature film, or to include trailers from the same film distributor that supplied the feature film.
There are two main types of trailers, the teaser and the typical trailer. Teasers are very short, normally bordering on thirty seconds, and only give a very small glimpse of the film. They normally try to set up an interest in the audience, and give them a very small idea of what the film is about. Teasers are very common for summer films, and are often released many months before the movie is released.
The standard trailer is two to three minutes long, and is done with a lot of editing. The cuts are quick, and try to give a fuller sense of the film than a teaser might. It normally advertises the stars, music, director, and/or writer. The trailer also gives an idea of the genre that it is in. Often the trailer is made with scenes that are later cut from the film. The music is also not always the music that shall be in the film, for most music is made in postproduction.
DVDs constantly contain the trailers of their films as a special feature. A nice feature that allows one to see how the film was originally advertised and what scenes were left on the cutting floor yet used in the trailer.