understanding the difference between a trailer and a opening sequence

The purpose of an opening sequence:

The main purposes of a movie opening sequence is to draw in the interest of the audience by setting a mood or atmosphere, which allows the audience to relax and forget about the real world for a few hours. Also, the opening is used to give the audience information about what will happen in the film, but not too much to give away the ending to early; making the audience want to find out what is going on in this film.

the conventions of a opening sequence:

• it tell the audience who the main actors are it the film
• it may give a setting of where the film is based
• the opening must be related to some part of the film, or it would be pointless to have one
• giving small amounts of info about the film
• it will be manly close-ups and ex-close-ups, limiting what the audience can see
• will be used as a base for the film to start off

The purpose of a trailer:

The main purpose of a movie trailer is to sell the film to people, making people want to see the film. Also, the trailer is used to show people the best bits from the film, similarly making people want to see the film; because they what to see what happens as a result of act.

The conventions of a trailer:

• gives info about the film (genre, actors, etc)
• it will give a brief idea of the plot of the film
• it is show off the best/funny bits of the film, to make people want to see why this is happening
• it uses very short, quick shots
• voice overs