With the EPUB file extension digital books are marked in the Open Ebook format. It is an open, XML-based standard for digital and electronic publications. This is developed by the International Digital Publishing Forum (IDPF).
The EPUB format replaces the older standard Open eBook (EOB) and can contain not only the book data but also DRM (Digital Rights Management) mechanisms.
The EPUB format is based on three open standards, all developed and maintained by the IDPF.
Open Publication Structure (OPS), which defines the formatting of the actual content
Open Packaging Format (OPF), which defines the structure (including table of contents)
Open Container Format (OCF), which regulates the packaging of all necessary files to a ZIP archive with EPUB extension
epub files can be viewed with many readers and some readers, for example OYO, Apple iPad or Sony Reader. In addition, there are a variety of conversion programs to convert EPUB into other formats.
Most often resolving problems with opening EPUB files is very simple. Just install an appropriate program that supports such files. All of the listed programs support EPUB files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of EPUB files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate EPUB files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Adobe Digital Editions applications is not on the list, select "Browse" option in order to manually locate the directory where Adobe Digital Editions has been installed.
Sometimes problems with opening EPUB files may not lie with the application itself, but can arise due to other causes, such as: