Short clips taken from parts of the film. Often used to help with the advertising of a new film. They are usually released before the actual trailer, and often interest people.
Example: Jurrasic Park-Had a couple of teaser trailers, which didn't give away too much information, but allowed the audience to know what it related to.
Trailers are a series of clips taken from the film wich are put together to make a 'short film'. They often show the funniest or more serious parts of the film(the best bits), however, they do not give too much information away, so that the audience go and see it at the cinema.
Example: Pirates of The Caribbean: On Stranger Tides showed one of the funniest scenes to capture people's attention, making them want to see what that part was about (the part where Captain Jack Sparrow is screaming).
The stars of a film are the Actors/actresses used. They play the characters. The bigger the star, the more popular the film will probably be.
Example: The Die Hard films are very popular due to Bruce Willis being the main role in all of them.