SHTML is an HTML file that includes server instructions or server side includes and is similar to an ASP file. 
SHTML. A file extension that is also sometimes known as SSI, SHTML is an HTML file that includes server instructions or server side includes and is similar to an ASP file. This file is used to associate files that include server instructions, so all files load as fast as possible. A server administrator may set the name of the file extensions to anything. However, the files are commonly either .SHTML or .SSI. SHTML is a file extension that lets the web server know the file should be processed as using Server Side Includes (SSI). (HTML is...you know what it is, and DHTML is Microsoft's name for Javascript+HTML+CSS or something). This Hypertext Transfer Protocol facility is referred to as a server-side include. (Although rarely done, the server administrator can identify some other file name suffix than .shtml as a server-side include file.) You can think of a server-side include as a limited form of common gateway interface application. The SHTML files are written in the HTML language and are inserted into an HTML website. These files are then processed by a Web server before being displayed on a user's Web browser. The server then adds the information in the file “on the fly” when the user is browsing the site that the file is associated with. shtml isn't really anything much. It's a file extension (*.shtml) that is commonly used by servers to recognise an html file which includes something called Server Side Includes (SSI). Server Side Includes basically let you dump the contents of a file straight into an html document, if your server allows them.
http://www.computerhope.com/jargon/s/shtml.htm http://stackoverflow.com/questions/519619/what-is-shtml http://searchsoa.techtarget.com/definition/shtml http://file.org/extension/shtml http://html-faq.com/ssi/?shtml
what is shtml