EPUB (from Wikipedia) (short for electronic publication; alternatively capitalized as ePub, ePUB, EPub, or epub, with "EPUB" preferred by the vendor) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub.
You can read more about this format:
Helpinator allows you to create EPUB books with one mouse click. All you have to do is:
1. Select template for this format in project options.
2. Install one of the available EPUB viewers to preview compiled ebook. For example you can use CoolReader:
HTML Templates for EPUB/MOBI formats are different from those used in CHM/WebHelp/QtHelp/JavaHelp. Though they too have <templatename.html> and <templatename.css> files, their logical structure is different: