Teasers also generally come out further away from the actual release of the film then trailers. For example, there was a teaser for Wall-E in 2007, almost a full year before the movie was released. Dark Knight also had a teaser well in advance. Teasers, as someone said, are generally shorter and are meant to build pre-awareness. Marketing-wise, this is quite different from the purpose of a trailer. Teasers also frequently are made before the movie is finished completely, and thusly contain very little actual footage from the movie itself, and sometimes even footage that eventually is cut from the final film, or footage that is created just for the teaser. They generally stay away from any specific plot points, since the movie usually isn’t done yet. Sometimes, the movie even goes through major tonal shifts after the teaser is released. A good example of a current teaser is for Terminator Salvation.