This topic gives an overview of HTML Help from two perspectives:
From an author's standpoint, standard HTML content is supplemented with two navigation files and a project file. Run them all through the HTML Help compiler and you end up with a single, compressed file having a .chm file extension.
On the viewing side, .chm files utilize the IE layout engine (Shdocvw.dll) along with HTML Help's ActiveX control (Hhctrl.ocx) and a few small add-ons. An API is also provided so that an application can communicate with help.