Activity modeling focuses on the execution and flow of the behavior of a system, rather than how it is assembled. Possibly more than any other UML diagram, activity diagrams apply to much more than just software modeling. They are applicable to just about any type of behavioral modeling; for example, business processes, software processes, or workflows. Activity diagrams capture activities that are made up of smaller actions
When used for software modeling, activities typically represent a behavior invoked as a result of a method call.
An activity is a behavior that is factored into one or more actions. An action represents a single step within an activity where data manipulation or processing occurs in a modeled system.