Activity diagrams are basically graphical representations of use case descriptions. For each use case identified, you can diagrammatically represent the exact flow of events within a use case. It is now easier to see what action goes where and is performed by whom.
Activity diagrams can consist of many parts, namely:
Each of these parts are then then nicely wrapped up in Activity partitions.
When referring to an “activity”, I am referring to one diagram that describes a particular use case.