One of the most powerful tools for customizing SharePoint 2007 is master pages. One of the most common stumbling blocks is understanding the many content placeholders and what they’re used for.
There have been several articles published detailing the placeholder which has been a huge help. For those that deal with SharePoint Customization on a frequent basis, I’ve created a desktop based on the placeholder articles which you can set as your windows desktop and use as a quick-reference. (The descriptions are not the greatest so I’ll update it again with better descriptions later). The following desktop outlines the default placeholders on a WSS 3.0 Master Page: