There are two main methods of including a file:
<!--# include file="file.html" -->
<!--# include virtual="/path/to/file.html" -->
the first method allows you to include a file relative to the .shtml file calling the directive.
the second method allows you to include a file based on the web root, where the above example would correspond to the file:
http://yourdomain.com/path/to/file.html