A timeline is basically a table of CVSTrac objects. Normally, these objects are broken up by day or by milestone. Each object is dated, has some kind of identifying icon, a description of the object, some detail about the change, the name of the user responsible, and, if you have sufficient permissions, a link to a more detailed view of the object.
At the bottom of the page is a grouping of controls to change various timeline settings.
Most of the timeline toggles are about filtering what objects are seen. It's also possible to control the time window shown in the timeline by adjusting the number of days and the end date/time.
If you change any timeline settings, the new settings are stored in a browser cookie. Additionally, a user with admin settings can force his/her current timeline settings to be the default for all other users.
The timeline is available as an RSS feed. Just follow the "RSS" link if the site admin doesn't have feed auto-discovery enabled.