To view the history of a task right-click the task and select 'Properties'. Or you can double-click the task. Go to the 'History' tab.
The history shows every action which has been performed on the task. The 'Action' column described what action was performed. The 'User' column shows which user performed the action. The 'Time' column shows the date and time when the action was performed.
If the action is a changeset action then you can right-click on the action and select 'View Changeset' to open the corresponding Changeset Dialog.