With the file extension SSI, HTML files are marked with server side includes (SSI). These are special html files that contain the instructions (script commands) that the web server should execute before the finished page is sent to the client, ie browser.
The instructions usually beginning with '<! -' are ignored by most servers on files ending in htm or html. A typical application is the integration of date and time.
The extension seems to be mainly used or recognized by Dreamweaver. Much more commonly used endings are shtm, shtml and sht.
SSI files can be used like html files in any text editor and web page editor, but they can only be used by SSI capable servers, for example Apache or Microsoft IIS.