An activity diagram is a type of flowchart that shows the flow from one activity to another in a system or process. It is a graphical representation of workflows of stepwise activities and actions with support for choice, iteration, and concurrency. Activity diagrams are intended to model both computational and organizational processes, as well as the data flows intersecting with the related activities. They are used to describe the different dynamic aspects of a system, including how activities are coordinated to provide a service at different levels of abstraction.
Activity diagrams are constructed from a limited number of shapes, connected with arrows. The most important shape types include stadia, which represent actions; diamonds, which represent decisions; bars, which represent the start (split) or end (join) of concurrent activities; a black circle, which represents the start (initial node) of the workflow; and an encircled black circle, which represents the end (final node) . Arrows run from the start towards the end and represent the order in which activities happen.
Activity diagrams can be used to model software architecture elements within a system by showing the relationships between the components and the constraints for each component. They can also be used to illustrate a business process or workflow between users and the system, and to simplify and improve any process by clarifying complicated use cases.
