A great article of Liam Cleary (Solution Architect) on MOSS 2007.
...MOSS 2007 as your should be aware has been changed slightly with it now being built as effectively an extremely advance ASP.NET 2.0 Application. Each of the pages has now been styled with CSS as before but now the layout is decided b using Master Pages and Page Layouts. This way of customisation gives you a quick and easy way to change your Portal and Sites. The pages are now rendered as below....
Part: 2 = http://www.sharepointblogs.com/helloitsliam/archive/2006/08/30/11723.aspx
EROL MVP SPS