A teaser trailer is specifically that - one which is meant to tease. They are short and tend to last between 30-60 seconds and contain little actual footage from the film being advertised. Hence they do not reveal any significant content or plot details and are meant only to give a minor glimpse of what is to come. This literally 'teases' the audience for they are often released while the film is still in production or the editing phases and only disclose small information such as 'coming soon'.
Teaser trailers are usually only made for big-budget and popularly themed movies to add to the hype and buzz of the release through word of mouth. Thus, today they are increasingly focused on internet downloading, blogging and the convention circuit.
Example Teaser Trailer:
A theatrical trailer, on the other hand, is more detailed and longer, it normally lasts between 60-120 seconds. It may provide plenty of information as to what the plot is about and is shown in theaters right before the film's release date. Many modern trailers, in fact, seem to give away too much content ahead of time and thus ruin part of the surprise of the film.
Example Theatrical Trailer:
