See it all the time, but not totally sure what it means?
People are all the customer org’s workers. People can be assigned to services and will have time sheets connected to their accounts to populate the hours they are working towards an assignment.
Users are the individuals that have some capacity of access to Parallax. Users are the individuals actually utilizing Parallax. The level of access for a User is manage under Permissions.
Typically all Users will also be People...
.....but not all People will be Users.