The .CHM file extension belongs to a format created by Microsoft for the development of online help files. HTML Help Files are normally created with a help authoring tool, such as Microsoft's free "HTML Help Workshop".
A CHM help file contains a set of web pages and a hyperlinked table of contents allowing the reader to easily navigate to the required section of information/instructions.
While this format was designed specifically for encoding help files it is now also employed in the creation of some e-books or used for creating browsable archives of HTML pages.
* 2002 - Microsoft announced security risks associated with the .CHM format, Microsoft has decided to develop a new generation of windows help known as 'Microsoft assistance markup language' for Windows Vista to replace the CHM format.
** File type is susceptible to virus's and should be scanned before downloading.