Activity Diagrams describe the sequence of activities in a system with the help of Activities. Activity Diagrams are a special form of State Diagrams, that only (or mostly) contains Activities.
Activity Diagrams are similar to procedural Flux Diagrams, with the difference that all Activities are clearly attached to Objects.
Activity Diagrams are always associated to a Class, an Operation or a Use Case.
Activity Diagrams support sequential as well as parallel Activities. Parallel execution is represented via Fork/Wait icons, and for the Activities running in parallel, it is not important the order in which they are carried out (they can be executed at the same time or one after the other)
An Activity is a single step in a process. One Activity is one state in the system with internal activity and, at least, one outgoing transition. Activities can also have more than one outgoing transition if they have different conditions.
Activities can form hierarchies, this means that an Activity can be composed of several “detail” Activities, in which case the incoming and outgoing transitions should match the incoming and outgoing transitions of the detail diagram.
There are a few elements in UML that have no real semantic value for the model, but help to clarify parts of the diagram. These elements are
Text lines are useful to add short text information to a diagram. It is free-standing text and has no meaning to the Model itself.
Text Notes and anchors
Notes are useful to add more detailed information about an object or a specific situation. They have the great advantage that notes can be anchored to UML Elements to show that the note “belongs” to a specific object or situation.
Boxes are free-standing rectangles which can be used to group items together to make diagrams more readable. They have no logical meaning in the model.