Server Side Includes (SSI) is a simple server-side language, which allows you to include text from a specific source inside a website. In the most common scenario, the text from a file is included in another, providing a site the feeling that it is dynamic. For instance, if your site has 10 pages, five of them can include the content of any kind of file, like horoscope.txt. In case you change this text file, the new content is going to appear on all of the five webpages, which will let you update your site faster and easier than if you had to update a component of all five pages. Server Side Includes is sometimes used to incorporate the output of basic commands, scripts or functions as well - a hit counter that's shown on the site, the current time and date or the visitor's IP address. Every web page that makes use of SSI will need to have a unique extension - .shtml.