The title sequence is the first thing you see when the film starts, which means that it is one of the most important parts of the movie as it gives the audience the first impression of the film.
There are three basic structures which are followed:
A narrative opening with the titles running throughout; this occurs in the opening of films such as 'Panic Room' which has a helicopter flying over New York City with the titles being shown throughout the sequence on the sides of buildings and scenery. The camera shots that are used purposely focus on the titles.
A discrete title sequence; A discrete title sequence is a small, self contained film that may not have much relevance to the actual opening of the film but is used just to introduce the titles. For example, in the film 'Se7en' where the opening titles are of a psychotic man organising his unusual belongings, and then after that the opening of the film is completely different to the title sequence.
Titles over a blank screen, followed by the narrative opening; was used in the Donnie Darko title sequence.