This blog posting applies to Microsoft Office SharePoint Server 2007.
Many times it helps to see an overall comparison of SharePoint 2007 permissions. I assembled a matrix that shows lots of useful information that helps you understand how permission levels are related, and how they affect security trimming of Site Actions and Site Settings.
Screenshot (click to download the spreadsheet)
Update for SharePoint 2010
Colleague Rob Finney has updated this spreadsheet for 2010. Please see Rob's blog posting on the topic.