SHTML is a file extension that lets the web server know the file should be processed as using Server Side Includes (SSI).
It looks like SHTML = Server Side include HTML (SSI), which is just a fancy way of saying the server can dynamically inject code at the reference point.
SHTML is server-side scripting.
HTML is the language used to define a Web page. It looks like text, with instructions on how the web browser should display the page
Use static HTML when you want to have static content. Use SHTML if you want to do simple things such as includes.