Some events are triggered when a workflow is executed. The tracking service captures these events to record tracing information. A workflow can trigger the following three events:
1. workflow events: a workflow event indicates an event in which the workflow status changes. Each change in the workflow status triggers a trail event in the tracking service, workflow events are defined in the trackingworkflowevent enumeration and include the following values: created, completed, idle, suincluded, resumed, persisted, unloaded, loaded, exception, terminated, aborted, changed, and started.
2. Activity events: Activity events indicate changes in the activity status. The activity status is defined in the activityexecutionstatus enumeration, including initialized, executing, canceling, closed, compensating, and faulting.
3. user events: In workflow events and Activity events, we can only track defined events, sometimes we need the information at a specific position in the workflow. In this case, we can use a custom tracking point to track events anywhere throughout the workflow lifecycle. We use the trackdata () method of activity to create user tracking points.
Http://www.cnblogs.com/carysun/archive/2008/11/21/WFTrackingDB.html