Teaser trailers vs Trailers

Teaser trailers

According to Wikipedia: Teasers, unlike typical theatrical trailers, are usually very short in length (between 30–60 seconds) and usually contain little, if any, actual footage from the film. Sometimes, it is merely a truncated version of a theatrical trailer. They are usually released long in advance of the film they advertise. One of the reasons for the name 'teaser' is because they are shown usually a long time (one or one and a half years) before the movie comes out, so as to 'tease' the audience.

Trailers

According to Wikipedia: Trailers or previews are film advertisements for feature films that will be exhibited in the future at a cinema, on whose screen they are shown. The term 'trailer' comes from their having originally been shown at the end of a film program. Besides in front of theatrical releases, movie trailers have now become extremely popular on the internet. Of some 10-billion videos watched online annually, movie trailers rank #3, after news and user-created video.
Trailers consist of a series of selected shots from the film being advertised. Since the purpose of the trailer is to attract an audience to the film, these excerpts are usually drawn from the most exciting, funny, or otherwise noteworthy parts of the film but in abbreviated form and usually without producing spoilers. For this purpose the scenes are not necessarily in the order in which they appear in the film. A trailer has to achieve that in less than two and a half minutes, the maximum length allowed by theaters. Each studio or distributor is allowed to exceed this time limit once a year, if they feel it is necessary for a particular film.


So, in conclusion teaser trailers are far shorter than trailers and they contain quick shots of the film (which actually hasn't been filmed yet) to 'tease' the audience. There shouldn't be a story or anything too revealing about the film. Their purpose is less to tell the audience about a movie's content than simply to let them know that the movie is coming up in the near future, and to add to the hype of the upcoming release.