Activity diagram

In the Unified Modeling Language, an activity diagram represents the business and operational step-by-step workflows of components in a system. An activity diagram shows the overall flow of control.

Overview
In SysML the activity diagram has been extended to indicate flows among steps that convey physical element (e.g., gasoline) or energy (e.g., torque, pressure). Additional changes allow the diagram to better support continuous behaviors and continuous data flows.

In UML 1.x, an activity diagram is a variation of the UML State diagram in which the "states" represent activities, and the transitions represent the completion of those activities.

The UML 2.0 activity diagram, while similar looking to the UML 1.x activity diagram, has semantics based on Petri nets. In UML 2.0, the interaction overview diagram is based on the activity diagram.

Construction
Activity diagrams are typically used for business process modeling. They consist of:

- Initial node.
- Activity final node.
- Activities

The starting point of the diagram is the initial node, and the activity final node is the ending. An activity diagram can have zero or more activity final nodes. Inbetween activities are represented by rounded rectangles.

Reference:
http://en.wikipedia.org/wiki/Activity_diagram