Effort is the amount of actual work time required to complete a task.
Duration refers to the span of time during which the task will be completed, but is not necessarily the amount of work that the task requires. Effort can be different than Duration when a resource is assigned multiple tasks over the same time period.
Example:
Michael has been assigned 2 tasks for this week: Task A and Task B. He will work on Task A for 5 hours each day, and then work on Task B for the final 3 hours.
The Start and Due dates are the same Task A and Task B, and the Duration for both is 5 workdays.
The Effort for Task A is 25 hours (5 days x 5 hours per day) or 3 days, 1 hour.
The Effort for Task B is 15 hours (5 days x 3 hours per day) or 1 day, 7 hours.
Click File, click Options, and then click Task Info. In the dialog, check the option Display Effort field.
You can enter the Effort value for a task in the Task Info pane. The Effort defaults to the task Duration. To enter a different Effort value, un-check the link between Duration and Effort, and enter a new value.
Effort is only displayed for a task when it is different than the task's Duration.