Oliver Rist at InfoWorld - Preview: Windows Workflow Foundation:

“WWF creates a class of application that is rarely seen except when created through extraordinary effort: A distributed user-facing application.

From a developer’s perspective, WWF is a toolbox of abstractions for workflow-related activities such as receiving and sending Web services calls, taking conditional branches from an otherwise sequential workflow during the course of its execution, firing and sinking (receiving) asynchronous events and managing nested workflows. WWF abstracts and extends familiar paradigms in ways that change how developers think.”