| Vaguely standards-based documentation format designed by Microsoft. Used for user help in Windows 98 and mostly replaces WinHelp, the old hypertext documentation system in Windows.
HTML Help uses HTML documents, as opposed to WinHelp's compiled RTF files. It also uses a keywords and index file that can be parsed by Java and ActiveX controls to produce a pretty expanding/collapsing index environment for the end user. Each topic is a HTML file, and can contain any kind of HTML you want, including pictures, applets, CSS, scripting, or anything else you could do in a web page. You can also compress the keywords, index, and topics files into a single file called a CHM, that's usually only half as large as its uncompressed contents.
The catch? The default viewer is a group of ActiveX controls included in Internet Explorer and Windows 98 or newer. You can download the controls free of charge... if you are running Windows. Fat chance, Mac and Linux users. You can set up the same HTML pages with a Java applet, but you don't get compression or a few other features, and the Microsoft Java applet is "mysteriously" buggy compared to ActiveX. Also, many WinHelp features, such as pop-up text, annotations, and precise text control, are missing or difficult to implement. And the iteration required everyone to rethink their help strategy, and purchase another round of crap in the way of RoboHelp upgrades and training manuals. | 
