An Activity diagram is similar to a flowchart. Activity diagrams and Statechart diagrams are related. While a Statechart diagram focuses attention on an object undergoing a process (or on a process as an object), an Activity diagram focuses on the flow of activities involved in a single process. The Activity diagram shows how these single-process activities depend on one another. 

Activity diagrams can be divided into object swimlanes that determine which object is responsible for an activity.

The Activity Diagram below uses the following process: "Withdraw money from a bank account through an ATM." The three involved classes of the activity are Customer, ATM Machine, and Bank. The process begins at the black start circle at the top and ends at the concentric white/black stop circles at the bottom. The activities are shown as rounded rectangles.  

The three involved classes (people, and so on) of the activity are Customer, ATM, and Bank. The process begins at the black start circle at the top and ends at the concentric white/black stop circles at the bottom. The activities are shown as rounded rectangles.