What is a Mac user?

Depending on how you mean it, it's either some one who "uses" a Mac or a Mac that has a "User" account.
When you first log into a Mac you make a "User" account so it can be separated from other people that use it, keeps them from accessing your stuff. Jane and Bob both use one Mac so Jane and Bob have there own accounts, Jane has a separate user account so Bob can't mess with Janes settings or access her other stuff.