Smart Webby's Guide: Server Side Includes
Server Side Includes allow you to write some commonly used code once and have the server insert it into the pages for you. They cut down your development time by more than 100% as you only need to make changes in the include file and it automatically gets updated in all the pages that use the include. Best of all it reduces our FTP time as you only need to FTP one file and not all the pages using the Include. They are a huge time savers. This guide describes what SSIs are, and how to use them.