
Start Date is when the work item is expected to begin.
Due Date is when the work item is expected to end.
Actual End Date is the day the work item was actually completed. By default this is set to be the day the work item is marked as completed.
Actual Start Date is the day the work item was actually started. There is no out of the box workflow behind this field, it was designed to be manually entered. However, we recently released an app that will set the Actual Start Date to either the first day time is reported, the first day actual effort is reported, or the first day % completion is reported (whichever comes first). You can find the app here: http://www.clarizen.com/appsmarketplace/Set-Actual-Start-Date.html