Microsoft HTML Help format

Microsoft Compiled HTML Help is a proprietary format for online help files, developed by Microsoft and first released in 1997 as a successor to the Microsoft WinHelp format. It was first introduced with the release of Windows 98, and is still supported and distributed through Windows XP and Vista platforms.

The format has the following advantages over other systems:

• File size smaller than plain HTML
• It consists of a single compiled file (CHM) that can be installed locally on the users' PCs.
• Users do not have to have a network connection
• Range of formatting options that HTML gives for text presentation
• Ability to search the full text
• Ability to assemble several CHM files into one file with common TOC, index and search
• Ability to Generate TOC and Topic Folders containing international characters. Standard HTML Help will not generate these correctly.

(Main source : Wikipedia)