Forum Moderators: not2easy
<!--#include file="topmenu.html" -->
topmenu.html in this example would be the html needed for the top menu bar and this file could be easily modified affecting all pages that use it instantly.
Also, typically you use the file extention .shtml (for the main file calling the html) and have your hosting company allow certain code within these files to run. If you just tell your hosting company you have .shtml files with SSI's in them, they will set the correct server parameters so it all works.