Windows Workflow is a runtime and not an application. A host process must load and launch the workflow runtime before starting a workflow. The host process tells the runtime the types of workflows to create, and the runtime manages the life cycle of the workflows and notifies the host process about important life cycle events, such as workflow completion and termination. The runtime isn't particular about the type of host it lives inside. The host process could be a smart client application running on an office desktop machine, or an ASP.NET worker process running on a server in the rack of a data center. All the host processes needs is the ability to load the .NET 3.0 Workflow assemblies.
A host can also customize the workflow runtime by layering additional services on top of the runtime's base feature set. These services can provide persistence support for long-running workflows, tracking support for monitoring workflow execution, and more. Recall the ExternalDataExchangeService...