Workflow Activity Model
Windows Workflow Foundation (WF) in .NET Framework version 4 changes several development paradigms from previous versions; Workflows are now easier to create, execute, and maintain, and implement a host of new functionality.
The activity is now the base unit of creating a workflow, rather than using the SequentialWorkflowActivity or StatemachineWorkflowActivity classes.
The Go With The Flow blog has a posted a great 2-part story focused around working with the Workflow Foundation 4.0 Activity Model:
Workflow Foundation 4.0 Activity Model (I)
Workflow Foundation 4.0 Activity Model (II)
For more information about Workflow 4.0:
What's New in Windows Workflow Foundation