What is a teaser trailer?

In my experience teaser trailers come out a fair while before the initial film, particularly with sequels. Most recently this has happened with the teaser trailer for 'The Hunger Games' sequel 'Catching Fire'; the teaser came out roughly a month ago (May) despite the film not being due for release until November. 

Teaser trailers begin to initially introduce the potential audience to the film which starts to slowly build up the audience for the film. This is particularly effective when teasers go viral as it reaches a vastly wide audience in a very limited amount of time, perhaps even a wider audience than the actual trailer will end up reaching. 

In my opinion, teaser trailers are very good at creating a lasting impression with those who view it as it captures their attention more due to the shorter length, this then means that they will recognise the full trailer when it comes out and are more likely to want to watch the film due to the sense of familiarity.