ePub is an open format defined by the Open eBook Forum of the International Digital Publishing Forum (IDPF). It is based on XHTML and XML along with optional CSS style sheets. Its predecessor was the OEB standard.
Quoted from the IDPF web site:
- '.epub' is the file extension of an XML format for reflowable digital books and publications. '.epub' is composed of three open standards, the Open Publication Structure (OPS), Open Packaging Format (OPF) and Open Container Format (OCF), produced by the IDPF. '.epub' allows publishers to produce and send a single digital publication file through distribution and offers consumers interoperability between software/hardware for unencrypted reflowable digital books and other publications. The Open eBook Publication Structure or 'OEB', originally produced in 1999, is the precursor to OPS.
The intent of ePub is to serve both as a source file format and an end user format. For this reason the files are collected into a container for easy dissemination and use. This container is generally a zip file but the extension has been renamed to .epub. It has special requirements by including an uncompressed mime type file while the rest of the data in the file is compressed. An ePub reader should be capable of reading the content in its compressed format. http://wiki.mobileread.com/wiki/EPUB
The IDPF specification page contains the specifications for this format. In particular check the version 2.01 OPS and OPF specifications and the version 1.01 OCF specifications. The informational documents are also quite useful in understanding the standard's intent and content.
ePub version 3 is the newest version of the standard and has now been recommended by the idpf standards committee.
In version 2.01 there were three defining documents, the OPF (Open Packaging Format), the OCF (Open Container Format), and the OPS (Open Publications Structure). The OPS referenced a DAISY standard for the NCX file. The new 3.0 standard has 4 defining documents with new names. The OPF becomes the ePub Publications standard. The OCF remains the same and the OPS received the most changes to become the ePub Content Documents. This now includes the old NCX specifications which are no longer used. A fourth document is concerned with Media Overlays and is a new feature of ePub version 3. http://wiki.mobileread.com/wiki/EPub_3