ePub is an open publishing standard for electronic books put out by the International Digital Publishing Forum(IDPF). It consists of three component standards:
- Open Publication Structure(OPS)
- Open Packaging Format(OPF)
- Open Container Format(OCF).
It is XML(eXtensible Markup Language) based, and allows for reflowable digital books and publications.
The OPF defines the mechanism by which various components of an OPS publication are tied together.
- It lists all markup files, images, navigation structure, etc.
- It provides publication metadata.
The OPS is the standard for representing the content of electronic publications. Specifically, standards and guidelines that ensure fidelity, accuracy, and accessibility, and adequate presentation of the content on various reading platforms. Content will adjust itself depending on the type of device it is displayed on.
The OCF defines the rules for collecting a series of related files into a single file container. ePub publications are distributed as a single file archive which contains multiple files.
ePub supports, by design, Digital Rights Management(DRM), allowing it to prevent it being read on unauthorized devices. Whether or not DRM should be used on books is a topic for another day, but being as many publishers insist on it in order to release their materials electronically, for now, it is better than nothing. However, since it does not mandate a particular DRM scheme, different schemes may lead to device incompatibility with what is supposed to be a standard format. This is being discussed in committee.
The format, by allowing for reflowable text, does not necessarily factor in publications that require precise layout or specific formatting. There are also limitations on annotation in books, allowing easy transfer of attached notes.
Most major electronic readers, with the exception of the popular Amazon Kindle, support the format. It is hardly perfect, but as it continues to evolve, it is the best we have right now.