Read Chapter 7 Managing User Accounts on drupal.com for an in-depth explanation on Roles and user Permissions. As stated in the Glossary of Drupal terms, the Roles are a named set of permissions that can be applied to a user account. While permissions are the ability to perform some action on the site, such as editing a particular type of content, or even the ability to see a particular page.
Roles
Roles are a named set of permissions that can be applied to a user account. Instead of deciding what permissions each user with access to your site needs (which can get tedious if many individuals use or visit a site), assigning them a role gives them all the permissions assigned to that role. Like with real life job functions, an administrator might have different needs and responsibilities than an editor, a site’s writing team, or someone doing research. By default there are 3 Roles for every site built with the UMD Terp templates, Anonymous users, Authenticated users, and Administrators (or “Admin”).
Permissions
Permissions let you control what users can do and see on your site. You can define a specific set of permissions for each role. See the Roles section to create a role. Any permissions granted to the Authenticated user role will be given to any user who is logged in to your site.
Warning: A number of permission settings have serious security implications for a site. Permissions should be maintained by trusted individuals with extensive knowledge in developing for Drupal sites.
User Accounts
Anyone who visits the site is considered a user, even you, reading this documentation, are considered a user. There are three default roles/groups of users when a Drupal site is created. User accounts provide an individual with an identity to the site, and when assigned roles (like administrator), allow those individuals to add or edit content, make changes, or any number of the activities described in this documentation.