I just saw a good article about groups in Windows 2000 and up on askthemct.com. The questions answered by the article came up in our Windows 2003, 70-290 study group.
Basically, he explains which groups can be nested, and this can be a huge issue when you are just getting started in the newer Active Directory stuff.
Here’s a link to the article.
http://www.askthemct.com/2008/01/20/groups-groups-and-more-groups/