Essentially, today any preview for an upcoming movie is referred to as a trailer.
The term “trailer” originally applied specifically to the actual film (not the content, but the 35mm or 70mm strip of celluloid) that is attached to the beginning of a 35mm or 70mm feature to advertise anything from the theater chain to theater policies to concession items to sound systems to previews of coming attractions. In theaters that still project their presentations on film, rather than via digital projectors, trailers are still used.
As digital media became more popular, people began referring to “previews” as trailers, and today, the terms are synonymous.
Studios engage various strategies to promote upcoming features, one of which is the preview – or trailer. Often multiple previews are created for the same film. These may be designed to appeal to different demographics, to be paired with specific feature films, or to spark curiosity in viewers far in advance of the release date. Teaser posters (or one-sheets) are also often released for the same purpose.
A teaser trailer is extremely short and generally shows very little of the film being advertised. Its purpose is to peek interest. Often teasers are created before production of a film is finished, and they have been known to contain images or segments that do not appear in the final cut. Other times, a teaser may be entirely made up of footage that does not appear in the movie being advertised.
A regular trailer is generally two or two-and-a-half minutes long. Different trailers for the same movie may focus more on different aspects of the film being advertised – one might prominently feature a lead actor, while another might focus more on jokes or dramatic moments.
During the first two weeks (sometimes longer) a movie is out, the trailers attached to the beginning are dictated by contract. Studios pay for that advertising time, just as brands pay for advertising time during the Oscars or the Superbowl. Theaters are required to not only attach the applicable trailers, but often attach them in a specific order.
Distributors hire freelancers to go to theaters and both confirm that the correct trailers are attached in the correct order and observe audience reactions to certain trailers. They use the audience reaction data for various purposes, including negotiating booking terms for the feature and developing additional trailers.