Workflow model can be written either in pure .NET code, pure XAML or Mix of XAML and .NET Code. A workflow model is compiled and can execute under windows, ASP.NET, Web services or windows services application.
What is a Workflow?
A Workflow is a set of activities which is stored as model and they depict a process. Below figure depicts clearly the difference between Workflow and Activity. Every task is an activity and group of activity depicts a complete workflow. Workflow is run by the Workflow runtime engine.