A brief outline of the plot (often told by title scenes or by dialogue of the characters) It doesn't tell you the ending to the film it just gives a selection of various action to make the audience keen to watch the film.
Introduces the main characters to the audience and lets the audience relate to them.
Shows the audience where the main settings of the film are.
Tells you who made the film and often what the producers have previously made.
Gives the audience the name of the film and when the release date is and if their are any famous actors in the film it tells you that too.
Typically trailers are 1 to 2 minutes long
Often has music playing the background, the music if often associated with the genre. E.G. action films have music upbeat music to create tension.
The main aim of a film trailer is to make the audience want to go and see the film. It gives the audience information about the plot to make the audience interested, they do this; introducing the main characters and shows funny bits or action bits depending on the genre.