Joomla Access Control Level
Joomla has a very flexible access control level system. But that also means that it is complex.
Aside from the Public group which is just your normal site visitor, Joomla has four user groups for access to the front-end: Registered, Author, Editor, and Publisher (listed in order from lowest authority to highest authority). These users login via the front-end login form (which can be located in a hidden location).
Joomla has three user groups for access to the back-end administration: Manager, Administrator, and Super User (listed in order from lowest to highest authority). These users login via the backend admin control panel access at for example: http://www.exmaple.com/administrator These users can login via the front-end as well. But when they do so, they have the authority of a front-end publisher.
User members are placed into one of these groups. When they are done so, they get the authority of that group. For example, users in the Registered group can not edit nor publish content, but they may have access to view restricted content. Publishers have full authority over what content is shown in the site and full authority to edit any content. Super User has the ability to basically do everything, including installation of plugins and system configurations.
Learn more about access control groups of Joomla 1.5 in this article on the Joomla forum.
With Joomla 1.6, you can unlimited user-defined groups and an user can be assigned to multiple groups. If you install the Joomla sample data, you’ll get the user-defined group of “Shop Suppliers” and “Customer Group” for example.
Beside group, there are also Access Levels. In Joomla 1.6, you can have unlimited user-defined access levels. Whereas in Joomla 1.5 you have three fixed Access Levels (Public, Registered, Special). Groups are assigned to Access Levels.
And then there are “Permissions” of which there are four: Not Set, Inherit, Deny, Allow.