chm The CHM file format and its associated .chm filename extension, is a proprietary file format developed by Microsoft. Its full name is compiled HTML Help. CHM was released to the public in 1997 as a bundle with Microsoft Windows 98 and was primarily designed to facilitate documentation on the Windows platform, particularly help files for software applications that run on the Microsoft Windows platform. Essentially, a CHM file is a collection of HTML files packed as a single compressed file. Though the format is proprietary, it was successfully reverse engineered by the late 90's to early 2000s allowing the development of open source readers that are capable of opening files with the .chm extension. The format was succeeded by the LIT format.
epub The ePub format is one of the many open source eBook file formats available. Initially released in 2007, it succeeded the legacy Open eBook format. The format was first defined by the Open eBook forum, part of the international digital publishing forum. EPUB digital books and publications all have the associated .epub extension and can be opened and viewed on any compatible device, software application, smartphone, or dedicated reader. The format is capable of optimizing the resolution of digital publications to suite the resolution of the device being used, making it inherently a 'reflowable' format.