Difference between a Teaser trailer and a theatrical trailer.

A trailer or preview is an advertisement for a feature  film 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 feature film screening.That practice did not last long, because patrons tended to leave the theater after the films ended, but the name has stuck. Trailers are now shown before the film (or the A movie in a double feature) begins.
A teaser trailer, or just teaser, is a short trailer used to advertise an upcoming film, television program, video game, or product.
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.
Teasers are also commonly used in advertising. The so-called teaser ad or teaser campaign typically consists of a series of small, cryptic, challenging advertisements which anticipate a large(r), full-blown campaign for a product launch or otherwise important event.
Tester trailers are usually only made for big-budget and popularly themed movies. 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. Teaser trailers are often made while the film is still in production or being edited and as a result they may feature scenes or alternate versions of scenes that are not in the finished film. Other ones (notably Pixar films) have scenes made for use in the trailer only. Teaser trailers today are increasingly focused on Internet downloading and the convention circuit.