The flexibility of the new ACL system has a downside: it can also get quite complex. The power to create as many user groups as you like, each with very fine-grained sets of permissions assigned to them, means you can easily get entangled in a web of user groups, Joomla! Objects, and permissions.
You should carefully plan the combinations of permissions you need to assign to different user groups. Before you change anything in Joomla!, sketch an outline or use mind mapping tools (such as http://bubbl.us) to get an overview of what you want to accomplish through Joomla! ACL: who (which users) should be able to see or do what in which parts of the site?
In many cases, you might not need to go beyond the default setup and just use the default users groups and permissions that are already present when you install Joomla! 1.6. So, before we go and find out how you can craft custom user groups and their distinctive sets of permissions, let's have...