The Workflow Web Service
The Workflow web service that ships with SharePoint is at once an incredibly powerful tool and quite possibly one of the most frustrating web services written. The workflow.asmx web service is used by client applications such as Office to determine if there are any outstanding tasks for a server-side document when that document is opened in Word or Excel or any other Office application. If there are outstanding tasks assigned to users who are viewing a document that has an active workflow, then those tasks appear below the ribbon in the Office client application. From there, the user can interact with the Office client to complete the workflow or advance it to the next step.
The difficulty with using the workflow.asmx web service is that many of the method parameters require intimate knowledge of the XML schema for the workflow itself. You can find some of the workflow data for installed workflows in the directory. Hopefully, if you are using this web service, you are using it as a client to a workflow that you created and you won't have to worry too much about the schema for the Workflow data and form parameters.
Conclusion
I've always been of the opinion that a good design is a design that you don't notice. The integration of WF with SharePoint is (in most cases) so seamless that end users don't notice that they're using a product that ships separately from SharePoint. In addition to the end-user experience, developers have a wide range of options for dealing with workflows in SharePoint. Regardless of your ultimate goal, if you are a SharePoint developer, then you can benefit immediately from learning the Workflow Foundation and experimenting with SharePoint's integration with Microsoft's powerful new framework.
