There is no web service to interact with workflows. (You could write a custom one I guess).
There is a Workflow Web Service: workflow.asmx, it is just undocumented for SP 2010.
What kind of operations are you trying to perform? That is a pretty broad area, and depending on the desired action, there may be different "better" approaches to take.
If you're trying to initiate a workflow, one way you could achieve that would be to set the workflow to start on the creation of a new list item (and hit that with Lists.asmx). This method also allows you to pass in initialization data in the list's columns.
If you're trying to act on a specific step in a workflow, I don't believe you'd be able to do so without writing a custom web service, or a custom workflow to act on your target.
Check out this similar question on SO: SharePoint, Workflow and Web Services - restart workflow