An activity diagram is a special case of a statechart diagram in which all of the states are action states and the flow of control is triggered by the completion of actions in the source state.
Related to a specific class or use case, an activity diagram describes the internal behavior of a method. Use an activity diagram to represent a flow driven by internally generated actions. Use a statechart diagram to represent a flow in response to external events.
Activity diagrams encourage you to notice and document parallel and concurrent activities. This makes them excellent tools for modeling workflow, analyzing use cases, and dealing with multi-threaded applications.