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.