Every visitor to your site, whether they have an account and log in or visit the site anonymously, is considered a user to Drupal. Every user also has a numeric user ID special to the type of user.
Types of Users
This user has the ID one (1). User of ID one (1) is the primary admin user account created during Drupal installation. This user is very special because it has permission to do absolutely everything on the site.
These users are assigned a user ID when they register for the website. A user name and email address is associated with any user that isn't anonymous (therefore must be logged in).
Anonymous users who visit the website but do not login all share a user ID of zero (0).
Associating more information with Users
Other information can also be associated with users by modules; for instance, if you use the core Profile module, you can define user profile fields to be associated with each user.
Other users on your site can be assigned permissions via roles. To do this, you first need to create a role by navigating to people --> permissions --> role. A common role is "Content editor" or "Member". Next, you will assign permissions to that role, to tell Drupal what that role can and can't do on the site. Finally, you will grant certain users on your site your new role, which will mean that when those users are logged in, Drupal will let them do the actions you gave that role permission to do.
You can also assign permissions for the special built-in roles of "anonymous user" (a user who is not logged in) and "authenticated user" (a user who is logged in, with no special role assignments). Drupal permissions are quite flexible—you are allowed to assign permission for any task to any role, depending on the needs of your site.
Read more about this topic in The Drupal Cookbook (for beginners).