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/campaign consists typically in (a series of) small, cryptic, challenging, advertisements which anticipate a large(r), full-blown campaign for a product launch or otherwise important event.
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 (notable Pixar films) have scenes made for use in the trailer only. Teaser trailers today are increasingly focused on internet downloading and the convention circuit.An early example of the teaser trailer was the one for the Superman film by Richard Donner.
The film was already nearly a year late; it was designed to re-invigorate interest in the release. The teaser for the Batman film starring Michael Keaton in 1989 was an emergency marketing move that successfully convinced angered comic book fans that the film would respect the source material.

irl packing clothes and her mum telling her to be careful, which gives the audience exposition, as to what may happen in the film- showing something could go wrong.