Server-Side Include is a type of HTML comment that directs the Web server to dynamically generate data for the Web page whenever it is requested.

Server Side Includes, when applied to an HTML document, provide for interactive real-time features such as echoing current time, conditional execution based on logical comparisons, querying or updating a database, sending an e-mail, etc., with no programming or CGI scripts.

An SSI consists of a special sequence of characters (called tokens) on an HTML page. As the page is sent from the HTTP server to the requesting client, the page is scanned by the server for these special tokens. When a token is found, the server interprets the data in the token and performs an action based on the token data.