A CHM help file has a ".chm" extension. It has a set of web pages written in a subset of HTML and a hyperlinked table of contents. CHM format is optimized for reading, as files are heavily indexed. All files are compressed together with LZX compression. Most CHM browsers have the capability to display a table of contents outside of the body text of the help file.
The file starts with bytes "ITSF" (in ASCII), for "Info-Tech Storage Format". The format has been partially reverse engineered and there are container and internal files specifications available.
On Windows computers, this help file can be compiled using hhc.exe, which is distributed at no charge with the HTML Help Workshop.
There are some open source tools which can read and explore these files (see for example xCHM, KchmViewer, GnoCHM, Chmox for OS X, Chamonix for OS X, Help Explorer Viewer, or the Firefox add-on CHM Reader), but they lack various features of the Microsoft Windows tools, most importantly a write support.
Chemdraw:
ChemDraw is a major molecule editor developed by the cheminformatics company CambridgeSoft. ChemDraw is part of the ChemOffice suite of programs and is available for Macintosh and Microsoft Windows. The program started as MacDraw, a simple Macintosh chemical structure drawing package.