It is now well known that the single most important improvement in Joomla! 1.6 is the newly designed and implemented Access Control. This is also the most discussed and expected feature for everyone using Joomla!
With Joomla! 1.6 Alpha available for people to conduct tests and get familiar with, there come a couple of good introduction articles on this much anticipated ACL feature. The first one, ACL Tutorial for Joomla! 1.6, was written by a Joomla! user and forum moderator, Mark Dexter. In the article he sort of establish some conventions how to describe the ACL mechanism in diagrams. These conventions were picked up by Andrew Eddie, a Joomla! Master Developer in the Core Team, when he wrote his Introducing the new permissions in Joomla! 1.6.
Mark's article focuses more on how the Access Control is presented in the system, while Andrew tries to explain how it is in action. They are both very helpful read in understanding this new Access Control in Joomla! 1.6.