I've seen those two concepts in Nuxeo Document Management. What is the main difference between sections and workspaces?
A Workspace is the area where you can create, modify, and generally work on documents. Access to a given workspace can be shared between a few (or more) users and groups of users to provide a collaborative work area.
A Section is the area where you publish finished documents in order to make them accessible to a large user base, maybe even make them public to unauthenticated users.
So you can see there are two main philosophical differences between them:
Of course all this can be customized by developers in any Nuxeo instance depending on use cases.