As you know in some complex services, WCF services will not be sufficient; Situation like online shopping in line transaction (the most famous sample!). you may choose workflow services in these kinds of cases to implement logic via work flow and expose it via windows communication services. With the .NET Framework 3.5, Microsoft has provided additional components to integrate workflows with (WCF) and The result of this integration is called workflow services.
Microsoft workflow like other workflows is implemented by activities and it is consist of two activities: sequence and flowchart. Sequence: activities are executed in sequence that is one after another. It's not possible to go backwards in sequence activities, however execution must always move forward. Flowchart: like sequence activity, activities are executed one after another in flowchart, but also allow control to return to an earlier step which makes it more flexible.
A workflow service must be hosted for it to respond to incoming messages. It hosted in the same way as WCF hosting which can be hosted in any managed application, under Internet Information Services (IIS).