Access Control Level (ACL)
Access Control Level governs the permissions that are associated with the individual User levels. There are six User Groups/levels within Joomla! Three only have access to the Site Front-end, Registered, Author, Editor, and Publisher. There are also three levels with access to the Administrator Back-end: Manager, Administrator, and Super Administrator. In addition to the registered User Groups listed above there is a Public (unregistered) or Guest, level.

• Registered User
  User level within Joomla! with access only to Site Front-end functions, though with permission to access parts of the Site that are not accessible to Public Users.

• Author
  User level within Joomla! with access to Site Front-end functions only

• Editor
  User level within Joomla! with access only to Site Front-end, authoring and editing functions.

• Publisher
  User level within Joomla!, with access only to Site Front-end functions, but with permission to publish content on the Site.

• Registered User
  User level within Joomla! with access only to Site Front-end functions, though with permission to access parts of the Site that are not accessible to Public Users.

From Joomla! 1.5 Glossary section of Joomla.org