An activity is a scheduled phase in a project plan with a distinct beginning and end. An activity usually contains several tasks upon completion of which the whole activity is completed. Several activities can be combined to form a summary activity. The duration of an activity is determined by the effort it takes to complete each of its designated tasks.
The start or end of an activity can also be determined by constraints, so called dependencies, between activities. Milestones, too, can determine the start or end date of an activity. Such dependencies between activities and other planning elements are visualized in a Gantt diagramm which is used by project managers to plan projects and monitor their progress.