It is a graphical representations of workflows of stepwise activities and actions with support for choice, iteration, and concurrency.
They are intended to model both computational and organizational processes (i.e. workflows).
- Activity diagrams show the overall flow of control.

The most important shape types:
- Rounded rectangles represent actions.
- Diamonds represent decisions.
- Bars represent the start (split) or end (join) of concurrent activities.
- Black circles represent the start (initial state) of the workflow.
- Encircled black circle represents the end (final state).
Arrows run from the start towards the end and represent the order in which activities happen.
- Hence they can be regarded as a form of the 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.

The Activity diagram models logic-any logic-from workflow to use cases to methods.
It borrows most of its notation from flowcharts but has added the concept of concurrency.
The arrows trace the flow from beginning to end through decisions and loops while identifying each logical step in the process.

Notations

Action: A step in the activity, in which the users or software perform some task.
- The action can start when a token has arrived at all its incoming flows.
Control Flow: A connector that shows the flow of control between actions.
Initial Node: Indicates the first action or actions in the activity.
Activity Final Node: An end to the activity.
Decision Node: A conditional branch in a flow.
- Has one input and two or more outputs.
- An incoming token emerges on just one of the outputs.
Guard: A condition that specifies whether a token can flow along with a connector.
Merge Node: Required to merge flows that were split with a decision node.
- Has two or more inputs and one output.
- A token on any input emerges on the output.
Comment: Provides additional information about elements to which it is linked.