In Joomla! 1.5, a fixed set of user groups was available, ranging from "Public" users (anyone with access to the frontend of the site) to "Super Administrators", allowed to log in to the backend and do anything. The ACL system in Joomla! 1.6 is much more flexible:
Instead of fixed user groups with fixed sets of permissions, you can create as many groups as you want and grant the people in those groups any combination of permissions. ACL enables you to control anything users can do on the site: log in, create, edit, delete, publish, unpublish, trash, archive, manage, or administer things.
Users are no longer limited to only one group: a user can belong to different groups at the same time. This allows you to give particular users both the set of permissions for one group and another group without having to create a third, combined set of permissions from the ground up.
Permissions no longer apply to the whole site as they did in Joomla! 1.5...