The differences between Teaser and Main Posters is that the teaser poster gives very little information about the film itself. The information that is usually given on a teaser poster is the release year and the title of the film. Also is shows a picture which generally represents the genre and hinting at what the film is about.
On the other hand a main poster usually gives information such as picture, tagline, credit block, use of star names, use of star, release date, producer/director and enigma. This gives the audience everything they need to know about the film.