Duration is the total amount of time in which the user has to complete the task. Duration is based on calendar dates.
In contrast, Work shows the actual time that is required to complete the task. For example, you might have a task that requires only 4 hours to be physically completed, but that task can be completed anytime over the next week.
Thus, in such case, the task work will be 4h, and the task duration will be 1w.