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) http://blogs.msdn.com/flow/archive/2010/01/24/lights-camera-activities-windows-workflow-foundation-s-activity-model.aspx
Workflow Foundation 4.0 Activity Model (II) http://blogs.msdn.com/flow/archive/2010/01/31/workflow-foundation-4-0-activity-model-ii.aspx
For more information about Workflow 4.0:
What's New in Windows Workflow Foundation http://msdn.microsoft.com/en-us/library/dd489410(VS.100).aspx