Access Control List
From Joomla! Documentation
The "J1.5" namespace is an archived namespace. This page contains information for a Joomla! version which is no longer supported. It exists only as a historical reference, it will not be improved and its content may be incomplete and/or contain broken links.
Overview of ACL in Versions 
|Version 1.5||2.5 series|
|Groups||7 fixed groups (Public, Registered, Author, Editor, Publisher, Manager, Administrator, and Super-Administrator)||Unlimited user-defined Groups|
|Users & Groups||A User can be assigned to only one group||A User can be assigned to multiple groups|
|Access Levels||3 fixed Access Levels (Public, Registered, Special)||Unlimited user-defined Access Levels|
|Access Levels & Groups||Relationship between Groups and Access Levels was fixed.||Groups are assigned to Access Levels. Any combination of Groups can be assigned to any Access Level.|
We see that in every case the ACL has been made much more flexible since , with unlimited Groups and Access Levels, and the ability to assign one User to multiple Groups and any Groups to any Access Level.