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 workspaces users own.

You can add collaborators to your workspace/folder while assigning them roles.

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

Reviewer - Has permission to only view the workspace. If you want to give permission to edit a diagram later on, you can simply change to Editor from the Share menu 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.

Find out more about Editors and Reviewers on this knowledge article What's the difference between an Editor and a Reviewer?