A web file type with the suffix of ".shtml" (rather than the usual ".html" or ".htm") indicates that the file includes some information that will be added "on the fly" by the server before it is sent to the users browser.
A typical use is to include the contents of a single file across many web pages or certain kinds of server variables.
This functionality is called Server Side Includes or SSI and is typically only supported by the ".shtml" file type.
