An Activity diagram is a visual representation of any system's activities and flows of data or decisions between activities. An Activity diagram shows the flow from one activity to another activity, it provides a flowchart of code. In addition, an Activity diagram can be a special case of a State diagram in which all or most transitions are triggered by the completion of activities in the source state.
When you open an Activity diagram in the Diagram editor, UML Modeling displays a specific set of icons in the Modeling Palette window. This is not the same set of icons displayed for other types of diagram.
UML Modeling uses an Activity diagram to provide a very broad view of a business process. This type of diagram might be used to break out the activities that occur within an Use Case. It typically shows many different activities that will be handled by different elements, and it can model parallel processing threads.