Who (user) did
When (date and time)
Where (IP address)
Tracking is mainly concerned with finding details about a subset of the data, such as an individual, an activity, or a course. An example is a pupil who insists he/she has submitted an assignment, which cannot be located. The tracking log will be able to shed light on this.
Moodle further offers Statistics mode, which provides a graphical summary about the number of hits in courses and the entire site.
If these monitoring facilities do not satisfy your hunger for usage data, you might consider using external web analysis tools such as web log analyzers...