Sharepoint workflow sits on the top of windows workflow and adds the human workflow flavor.
The first basic important point to note is Sharepoint workflow and windows are not different entities. The base for sharepoint workflow is windows workflow. In other words, Sharepoint workflow sits on the top of windows workflow and adds the human workflow flavor. The below figure reflects the broader definition of Sharepoint workflow. It takes the core windows workflow from .NET framework and then applies content centric workflow on it. Content centric workflows are nothing but simple windows workflow attached to lists, documents and custom contents.
You can visualize Sharepoint workflows as windows workflow with human workflow touch and feel. The human workflow touch and feel comes when the workflow is attached to a content type like list, document, etc.
For instance, let's say we have made approve and disapprove windows workflow. You can attach this workflow with a student content type and the human workflow for student then becomes student passed and student failed. If the content type is invoice, then approve workflow can become a invoice paid workflow and disapprove can become invoice not paid.
In other words, Sharepoint workflow is content centric and depicts human workflow.
