Server Side Includes (SSI) is a simple server-side language, which enables you to incorporate text from a specified source in a website. In the most typical case, the text from a file is incorporated in another, providing a website the feeling it is dynamic. For example, in case your site comprises of 10 webpages, 5 of them can contain the content of any kind of file, such as horoscope.txt. If you alter this text file, the updated content will appear on all of the five webpages, which will let you update your website much easier than if you had to update a component of all five pages. Server Side Includes is at times used to include the output of simple commands, scripts or functions as well - a hit counter that's shown on the site, the present time and date or the visitor's IP address. Any web page that employs SSI will need to have a unique extension - .shtml.
