Activity diagrams are graphical representations of workflows of stepwise activities and actions with support for choice, iteration and concurrency. In the Unified Modeling Language, activity diagrams are intended to model both computational and organizational processes (i.e. workflows). Activity diagrams show the overall flow of control.
Activity diagrams are constructed from a limited number of shapes, connected with arrows. The most important shape types:
rounded rectangles represent actions;
diamonds represent decisions;
bars represent the start (split) or end (join) of concurrent activities;
a black circle represents the start (initial state) of the workflow;
an encircled black circle represents the end (final state).
Arrows run from the start towards the end and represent the order in which activities happen.
Activity diagrams may be regarded as a form of flowchart. Typical flowchart techniques lack constructs for expressing concurrency. However, the join and split symbols in activity diagrams only resolve this for simple cases; the meaning of the model is not clear when they are arbitrarily combined with decisions or loops.