To Drupal, a role has the same meaning as it does outside of Drupal: the functional aspect of the user, sometimes considered a persona. It's worth mentioning that Drupal does blur the line a bit between a role and a user type, because it comes with three roles predefined, and they are the user types mentioned earlier. While administrator makes sense as a role, authenticated and anonymous are not really roles, but appear there to simplify things, for administrators, at least.
So, let's take a look at the default user roles inside of Drupal before we move on to discussing why you would want additional ones. If you are using the menus, click People in the Admin menu (admin/people), and then click the tab labeled Roles, as in the following:
The preceding screenshot shows the roles that are predefined in Drupal. I disagree with the given description of a role, because it is unclear. The order of operation in Drupal is that privileges are assigned to roles, and roles are assigned to users...