An onboardee is a new hire. A person that normally will only have access to the frontend (onboardee's dashboard).
Important remark: Always when creating an onboardee you are automatically creating an user attached to that onboardee account. However, if you create an user you don't create an onboardee automatically.
An user is anyone who is added to the app and an user can be and/or the following:
Administrator: Is someone with admin rights. This person is able to access the admin dashboard (which is always the app url followed by /companyadmin);
Key contact: This person not necessarily has access to the admin dashboard. The main goal of this person is to be used as a key contact for an onboardee.
Finally, as mentioned above an user can also be an onboardee.