Reusable workflows allow you to create a workflow backed by a SharePoint Content Type instead of an existing list or library. Once the content type is assigned to a list or library, the workflow can be associated with that same list or library. Unlike creating a List Workflow, a reusable workflow does require extra steps of associating the workflow with the list/library. The benefit is that the workflow can be used on as many lists/libraries that support the content type without having to re-create the workflow.
The following walk-through shows how to use SharePoint Designer to create a reusable workflow for the Document content type.