What is a teaser trailer?

A teaser trailer is a shortened trailer of the theatrical trailer, usually between 30-60 seconds and they contain very little of actual film footage or maybe none at all. They are released about a year to 18 months before the film is released and doesn't tell the date just maybe 'summer 2011', this may be because they haven't got a date set for release or maybe in some cases they haven't started filming.
Often in teaser trailers, the director of the film puts something for the audience to think about or speculate about (e.g. In super 8, what is inside the train.) This is very effective because it gets the audience to talk about it to different people who may have different views, which ultimately will create hype about the film and makes a snowball effect lasting until a new piece of information is released like a poster or even another teaser trailer. This is good for the big film companies because they won't have to spend huge amounts of money advertising the film on TV and in cinemas when the release date gets closer. Also the teaser trailer usually gives the genre of the film and not much else, nothing about the plot is released.
Teaser Trailers are normally made for big budgeted and popular themed films with the name of any well known actors or directors mentioned (e.g. in the Inception teaser it tells the audience that the director is Christopher Nolan, this is important because he is well known director and also it states another film he directed, The Dark Knight, and this is important because they know that The Dark Knight was very successful and had a very big following of very passionate fans, so they are trying to capture them and try to create a similar feeling for Nolan's new film. And also naming the main actor, Leonardo DiCaprio, who is a very famous name as the main actor.) JJ Abrams often doesn't put the name of the film on the teaser trailer usually just putting the date or the time of year, he does this because he has lots of fans through the tv programme Lost and mission impossible 3 so adds to the excitement that he is making a new film but nothimg has been released about it.