Could somebody explain the differences between content approval and document approval workflow in SharePoint 2010 and how they work together when they are both turned on in a document library?
The Document Approval workflow routes a document to a group of people for approval. The approval stage can be serial or parallel meaning approvers can do review in serial order (one at a time) or in a parallel order (all at once). Document Approval Workflows are created using out of the box workflow "Approval - SharePoint 2010" or can also be created as a custom SharePoint Designer Workflow.
The Approval Workflow can become as the Content Approval Workflow when Content Approval is enabled in the document library. Content Approval is enabled under List Settings > Versioning Settings > Require Content Approval for submitted items? "Yes" or "No".
In simple terms, when Content Approval is enabled, a document doesn't become visible to others until it’s published.
When creating a custom SharePoint Designer Workflow for your document approval there is an Action "Set Content Approval Status" that would update Content Approval Status in your document library depending on the status of the document.
The bottom line is there is no separate Content Approval Workflow in SharePoint but with Content Approval enabled in document library settings can turn a normal Document Approval Workflow to a Content Approval Workflow.
You can refer to below link to get info on Approval Work Flows: Understand approval workflows in SharePoint 2010