SQL Server 2005 System Tables
Microsoft has just released a diagram for the SQL Server 2005 System Views. It is a pretty good reference if you want to dig into some of the details of SQL Server. You can download the PDF here:
http://www.microsoft.com/downloads/details.aspx?familyid=2ec9e842-40be-4321-9b56-92fd3860fb32&displaylang=en