Server Side Includes
While standard HTML files are fine for storing pages, it is very useful to be able to create some content dynamically. For example, to add a footer or header to all files, or to insert document information such as last modified times automatically. This can be done with CGI, but that can be complex and requires programming or scripting skills. For simple dynamic documents there is an alternative: server-side-includes (SSI).
SSI lets you embed a number of special 'commands' into the HTML itself. When the server reads an SSI document, it looks for these commands and performs the necessary action. For example, there is an SSI command which inserts the document's last modification time. When the server reads a file with this command in, it replaces the command with the appropriate time.
You need to use the .shtml extension for your documents on WebTwon.net web servers.
You can get more information on SSI at ApacheWeek.