What's the difference between Users and Collaborators?

The main difference is, Users are the diagram owners whereas Collaborators are added by users to make changes/view documents users own.

You can add collaborators assigning roles to a document/folder you share with.

Editors - Who has the permission to view and/or edit the diagram. Editors can rename, comment, copy a diagram. In addition, they can share a diagram with an external user, remove shared users from the diagram, publish and unpublish diagram from the community, embed and export/import diagram.

Reviewer - Who has the permission to view only the diagram. If you want to give permission to edit a diagram later on, you can simply change to Editor and the permission will be updated immediately. The reviewer role is great for external users that don't require to edit but only to view the diagram.