An Activity Diagram is similar to the classic flow chart. It describes a workflow, business process, or algorithm by decomposing the flow of execution into a set of actions and sub activities joined by transitions and various connectors. These actions and sub activities are called activity nodes. An activity can be a simple linear sequence of activity nodes or it can be a complex series of parallel activity nodes with conditional branching and concurrency.

There will be multiple instances of this artifact. During requirements analysis, "black-box" activity diagrams are created describing the functional flows for the system as a whole. During architectural design, "white-box" activity diagrams are created that show the allocation of actions to specific system elements.

Activities may be grouped and assigned to objects – e.g. subsystems. In this case, the activity diagram is split into swim lanes that depict the respective responsibilities.