Microsoft Compiled HTML Help
"Microsoft Compiled HTML Help" is a document file format created by Microsoft in 1997. Files in the CHM format will have the file extension .chm. The CHM file format is essentially dead and nobody should use it for anything ever again. CHM is short for "Compiled HTML". It was meant to be used to store help text in Windows software. It was, during its prime-time which lasted from around 1999 to 2010, used to store and distribute a lot of e-books and other documents. Many older e-books about programming, the Internet and computer-related subjects in general are stored in the CHM format.
CHM files are archives with documents formatted using simpler mostly-standard early HTML. Documents can contain hyperlinks to other HTML documents within the archive. Documents can have embedded images.
The archives are compressed using a variant of the LZX algorithm.