SHTML is an HTML document (.shtml) that has Server Side Includes (SSI) in the document. These are generally found on Web server documents.
The most common use for this is to build a web page that includes several different sources of information. These sources can either be included verbatim or a script may build the inclusion from scratch. SSI capabilities include programmable scripting features sufficient to consider it a programming language in its own right. There are conditional commands and even formatting commands to create a well formed output.