What's The Difference Between Film Opening Sequences and Film Trailers?

Firstly, a 'Film Opening Sequence' is a sequence of scenes, usually shown at the beginning of a film. The Opening Sequence contains the names of the people who were most important within the production. The titles used within the opening sequence are usually placed over a scene which allows the audience to view them. The sequence builds an atmosphere for the audience, additionally, the sequence may perhaps enhance the plot building up the atmosphere for the actual film. On the other hand, a 'Trailer'  contains different clips, scenes and shots from the final movie. The Trailer contains the most attractive, exhilarating and enjoyable scenes from the film.
A Trailer is a combination of clips created for the films targeted audience. The Trailer aims to persuade potential audience members to go and watch the film once it has been released.
A trailer summarises the film, however, the trailer always ends when something thrilling or negative happens.

Here are the differences between both a Film Trailer and an Opening Sequence for the film: JUNO:

The Trailer:

                                                     

The Opening Sequence: