Update 11-22-2009: See this blog post for the SharePoint 2010 version of this information.
MOSS 2007 ships with many predefined Templates you may use to create Site Collections and sub sites. Sometimes the out of the box site Templates will meet the needs of your project. Other times, the out of the box site Templates may need to be enhanced.
Knowing which site Template has the functionality you need to deploy, or use as a baseline for a custom Site Definition, is one of the key decisions you make when architecting a SharePoint deployment. This being said, it is important to...