The purpose of a film trailer is for the production company to try and sell/ promote their film to an audience. It is a very important feature in the marketing stages of film production.
There are 3 types of film trailers. A teaser, a theatrical and a TV spot.
A teaser trailer is a very short trailer (30-60 seconds long) which is usually released a long time before the actual film is due to be released to tease the audience. They are only usually made for big budget productions. The film Paranormal Activity made excellent use of teaser trailers and made huge profits from the film.
A theatrical trailer is a much longer trailer (2-4 minutes long) and offer the viewer a much bigger insight into the film. They usually outline the story/ concept of the film and can be viewed on TV, online, internet or even in cinemas before films of a similar rating/ genre. They often include voice overs which introduce the film to the audience.
A TV spot is a trailer which is a short cut of a trailer which is shown on TV. Producers have to pay for this and air time is very expensive. They are designed to make a big impact on an audience in a short space of time.