Recently I came to know about SHTML. And also I have look at site having extensions as .shtml.
What is the purpose of SHTML in what ways it differ from HTML and DHTML
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).
You can use SSI to (for example) include a common header and footer in your pages, so you don't have to repeat code as much, and changing one included file updates all of your pages at once. You just put it in your HTML page as per normal.
It's embedded in a standard XML comment, and looks like this:
<!--#include virtual="top.shtml" -->
It's been largely superseded by other mechanisms, such as PHP includes, but some hosting packages still support it and nothing else.
You can read more in this article.
It’s just HTML with Server Side Includes.
来源：https://stackoverflow.com/questions/519619/what-is-shtml