Stapling Features to Site Definitions

http://www.etechplanet.com/blog/understanding-feature-stapling-in-sharepoint-7c-stapling-features-to-site-definitions.aspx

http://blogs.msdn.com/b/cjohnson/archive/2006/11/01/feature-stapling-in-wss-v3.aspx

http://msdn.microsoft.com/en-us/library/ff648422.aspx