Activity diagrams represent the business and operational workflows of a system. An Activity diagram is a dynamic diagram that shows the activity and the event that causes the object to be in the particular state.

The basic purposes of activity diagrams are similar to other four diagrams. It captures the dynamic behaviour of the system. Other four diagrams are used to show the message flow from one object to another but activity diagram is used to show message flow from one activity to another.

The activity diagram is suitable for modeling the activity flow of the system. An application can have multiple systems. Activity diagram also captures these systems and describes flow from one system to another. This specific usage is not available in other diagrams. These systems can be database, external queues or any other system.  

Following are the elements in Activity Diagram

Activity states, which represent the performance of a step within the workflow.

Transitions that show what activity state follows after another. This type of transition can be referred to as a completion transition. It differs from a transition in that it does not require an explicit trigger event; it is triggered by the completion of the activity that the activity state represents.

Decisions for which a set of guard conditions are defined. These guard conditions control which transition of a set of alternative transitions follows once the activity has been completed. You may also use the decision icon to show where the threads merge again. Decisions and guard conditions allow you to show alternative threads in the workflow of a business use case.

Synchronization bars, which you can use to show parallel subflows. Synchronization bars allow you to show concurrent threads in the workflow of a business use case.

The following is an example of an activity diagram for order management system. In the diagram four activities are identified which are associated with conditions.In the diagram four activities are identified which are associated with conditions.