Duration versus effort
Duration is how long a task takes in actual working hours in the schedule. If the task begins at 8:00 and ends at 12:00, that's a duration of 4 hours. Note that non-working times don't count; if a task starts one afternoon and doesn't complete until the next morning, or spans a lunch break, only the working hours are counted. Also note that the resources assigned to a task may have different working hours than the project as a whole, and thus the displayed duration of a task may not match up exactly with the amount of effort it takes.
Effort is the total amount of working time put into a task by all assigned resources. A task with a duration of 4 hours, with 2 resources assigned at 100%, has an effort value of 8 hours. Note that Material resources don't contribute to effort, only Staff and Equipment resources do.
The duration and the effort of a task can change depending on the resources you assign, and depending on the state of the 
icon in the Task Information inspector.
icon in the Task Information inspector. Determining which value to recalculate
The Effort and Duration fields in the Task Information inspector share an icon of two little resources .
. When you assign or unassign staff and equipment resources, the field with the darkened icon re-calculates its value; the other field stays the same. Click either icon to swap them. Regardless of which value is calculated, you can always edit either value manually.
All of the examples in the next section assume that the icon is on the Duration field, thus allowing the duration to be recalculated. Otherwise, the effort would be recalculated.
icon is on the Duration field, thus allowing the duration to be recalculated. Otherwise, the effort would be recalculated. What Makes Duration and Effort Change 
Number of Assigned Resources — Assigning more resources to a task generally causes the task to take less time, because the required effort gets completed sooner.
Efficiency of Assigned Resources — Assigning less-efficient resources to a task causes the duration to be longer than the effort, because the resource takes more time to achieve the same amount of effort.
Assigned Amount and Available Units — Assigning fewer units of a resource to a task causes the duration to be longer than the effort, because less of the resource's time and energy is being spent on that task. This situation is common when a resource is split between multiple tasks at one time. The amount of a resource you can assign to tasks depends on the resource's available Units (set in the Resource Information inspector or the Units column of the Resource outline). Assigning more of a resource than is available causes a violation and shows up as overutilization in the Resource timeline.