Server side includes (or SSI) is a set of tags which can be used within HTML pages to be replaced by something else, added ("included") by the server. An example might be that you have one file with copyright information which goes on the bottom of every page. By using a SSI tag, you could tell the server to replace every tag on every page with the copyright information. The benefit is that you could have one file containing the copyright information that gets placed on hundreds of pages on your site. By updating the single page, all the others are instantly updated when loaded by the server. On most servers you must use a filename extension of ".shtml" in order for SSI tags to operate.