A narrative opening is when the credits are played over the actual film, we are introduced somehow to either a theme a setting or characters or all. This is quite a common opening, but for a reason, as it is usually effective.
In this example, 'The Shining', the music contributes mostly to the mood:
To stress the importance of music, here is a fan-made video of the opening credits with a happy song, and the complete different atmosphere it creates.
Discrete Title Sequence.
A discrete title sequence is kind of like a film in itself. The images and shots shown are largely unrelated to the actual movie and it's plot. They usually display a more abstract view, and are solely used for mood, rather than information. An example is shown in the film 'se7en'.
Another good example is in the british horror/thriller 'Creep', for which i could not find a video.
The music and images set a mood but do not give away a plot.
'Donnie Darko' shows firstly a title sequence over a blank screen, before continuing with a narrative opening.