Activity Diagrams

Activity Diagrams show the work flow and they should be read from top to bottom.
When most other diagrams focus on objects, activity diagrams focus on activities and how messages are passed in between different activities.

An activity can be defined simply as an operation in the system.

So prior to drawing an activity diagram, it is required that we identify,

• Activities
• Associations
• Conditions
• Constraints

Activity diagrams cannot be matched with the code and are mostly used by business users only.

http://www.ibm.com/developerworks/rational/library/content/RationalEdge/jan02/t_activityDiagrams_fig1.gif

The above link provides a good explanation of the notations using a simple situation.