While menus are about providing access to the content on your disc, timelines are about the content itself, or packaging up the content in playable chunks. Once you create your menus and timelines, you then link them together into the interactive DVD presentation, as discussed in the following navigation section.
Compared to the design and interaction options available with menus, timelines are rather straightforward: one video track, with optional audio and subtitle tracks, and accessible via marked chapter points. But there still are some interesting possibilities for using timelines in unconventional ways.
Timelines have a video track, but they do not have to play video. A timeline can have no video, and still play audio, or display a still image, or display timed subtitles.
For an audio disc, when you want to use the DVD like a supersized CD jukebox, you can play the audio tracks with a blank video display. The advantage of the DVD, besides more music on the disc, is that you can add menus to access the music. You also can lay out and access the music both as chapters along the timeline and as alternate tracks, which can be particularly interesting for switching between different versions of the same piece.
And because it is a shame to totally waste the display, you might want to display a still image with information about the music, or use subtitles for optional informational overlays. Of course, you then can go further with ...