What's the difference between a trailer and a film opening?

Like mentioned on the brief and expectation post, we are expected to produce a film opening that LOOKS LIKE A FILM OPENING and not something else. Though this may sound ridiculous, it's possible to get a film trailer confused with an actual film opening. So what is the difference?

A FILM OPENING:

The most simple definition I can give of a film opening is the little bit that happens before the actual film, like an introduction; this includes the title sequence and company idents (e.g. Warner Bros. Pictures and Universal). A title sequence is the method by which cinematic films or television programs present their title, key production and cast members, or both, utilising conceptual visuals and sound. It usually follows but should not be confused with the opening credits, which are generally nothing more than a series of superimposed text.

Here is an example of a title sequence from the film Edward Scissorhands:

Here is an example of an opening of the film Edward Scissorhands: 

A FILM TRAILER:

Whereas, a trailer is a short promotional film composed of clips showing highlights of a movie due for release in the near future, which will be exhibited in the cinema. It's basically a preview of the movie to come. Trailers are now shown before the film begins to be shown or is released. 

Here is an example of a trailer of the film Edward Scissorhands: