In the context of a Content Management System (CMS), a user is simply someone who uses the site. If that were all there is to it, if everyone were always the same in the eyes of Drupal, we could stop right here. So, you won't be surprised to read that all users are not necessarily the same. Of course, I'm not referring to the personal aspects of the users when I write that; I mean that users are not necessarily all the same in terms of their reason for accessing the site and what ability they are given to do so.

If we look at USERS as a top-level classification with the thought of further classifying them, doing so depends on a choice: are we classifying them in a real-world sort of way, or as Drupal does? In the following table we see them compared: 

From the real-world point of view, we tend to categorize users based on their functional relationship with the site. If you think of these relationships in the context of a home, some might simply live there and consume food (think teenagers), some will contribute to the look and contents of the home, and others will be responsible for maintaining it. Yes, a user can appear in more than one category, as shown in the following table:

From the Drupal point of view, it pretty much comes down to what you will be allowed to do on the site and whether you require an account to do so. For example, some sites allow unknown site visitors to read the content and comment on it. Others only allow access to the content for them without commenting, and others do not allow anonymous access at all. Returning to the analogy of a home, those who can do whatever they like to it have administrative access; those who are allowed to enter have authenticated access, albeit with different "roles", such as friends versus residents; and those who come to the front door (if there is no gatekeeper or doorman preventing that) who are unknown are anonymous.