Activities are the elemental unit of a workflow. They are added to a workflow programmatically in a manner similar to adding XML DOM child nodes to a root node. When all the activities in a given flow path are finished running, the workflow instance is completed.
An activity can perform a single action, such as writing a value to a database, or it can be a composite activity and consist of a set of activities. Activities have two types of behavior: runtime and design time. The runtime behavior specifies the actions upon execution. The design-time behavior controls the appearance of the activity and its interaction while being displayed within the designer.
Windows Workflow Foundation contains a library of standard activities and provides the mechanisms for you to create your own. This enables extensibility and reusability between workflows.