The Windows Scripting Host serves as a controller of ActiveX Scripting engines, just as Microsoft Internet Explorer does. Because the scripting host is not a full Internet browser, it has a smaller memory footprint than Microsoft Internet Explorer; therefore, WSH is appropriate for performing simple, quick tasks.
The scripting host reads and passes the specified script file contents to the registered script engine via the IActiveScriptParse::ParseScriptText method provided by the script engine.
The scripting engine does not use the SCRIPT tag (used in HTML); instead it relies on the extension of the file. This way, the script writer doesn't have to be familiar with the exact ProgID of various script engines. The scripting host maintains a mapping of the script extensions to ProgIDs and uses the Windows association model to launch the appropriate engine.