Nothing -- they are the same!  Users and collaborators are the same thing and are used interchangeably. We tend to use the term "users" to designate the seats available in your plan, and the word "collaborators" to refer to the actual persons who join and contribute to your calendars, but they are really interchangeable notions. 

Each person that would be participating in a calendar (including clients who simply need to approve posts) is considered a user.

Learn how to invite a collaborator or user to your plan here. 

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