HTA is short for HTML Application, which are programs based on HTML and one or more scripting languages supported by Internet Explorer, usually VBScript or JScript. The default file-association for the .hta extension is the Microsoft HTML Application Host (mshta.exe).
If you have not disabled or changed this file association, in effect the HTA file behaves like an executable when double-clicked. An HTA runs as a fully trusted application and as a result has a lot more privileges than a normal HTML file.
