what is a Trailer?

Fast pass.
Jokes.
Best scenes.
Violent (sometimes).
Newspaper quotes.
Director.
Straight cuts.
Backing track.
Give the feeling of the film (sadness, fun, action, loss, etc.)
promote the film. ( makes you want to watch the film).
Generate interest in the film.
Show off the stars of the film.
Show the film to its best advantage. (whereas the opening scene won't show the best.)
creates excitement.
Tell people what the film is about e.g. the genre.
Not give too much of plot away.
Showcase some of the best bits of the film.
Give details about the production team.

i have also highlighted the conventions of a thriller's trailer so i don't get confused and i can look back on my notes for help.