An EPUB book is a digital book, or ebook, which can be read on a wide variety of digital devices including Tablets, Smartphones, Laptops and dedicated eReaders. It's this wide adoption that has allowed it to become the most dominant book format across the world, second only to Amazon's Kindle format in the U.S. and Europe.
EPUB is the successor to the Open eBook Standard, and became an official IDPF (International Digital Publishing Forum) specification in October 2007. In the spring of 2010 the IDPF made plans for the next iteration and after many months of hard deliberation they formally release EPUB3 in October 2011.
A discussion on the technology behind the specification would be a large undertaking but with the release of the EPUB 3 Best Practices book, I have decide to refer you to that. However, I would like to share a paragraph from the opening chapter as it gives a very good description;
One of the most common misconceptions about EPUB is that it is a “flavor” of XML. (“Should I use EPUB or DocBook?” or, even worse, “Should I use EPUB or HTML5?” Hint: EPUB (pretty much) = HTML5.) Due partly to the convenient single-file format provided as .epub, people sometimes fail to realize that EPUB is not just, and not mainly, a specification for the markup of content documents. It is a publication format, and as such it specifies and documents a host of things that publications need to include—content documents, style sheets, images, media, scripts, fonts, and more, as discussed in detail in the other chapters of this book. In fact, EPUB is sometimes thought of as “a website in a box,” though it is actually much more than that. -- EPUB3 Best Practices, 2013, O'Reilly.
The latest incarnation of the format is a much more involved specification than previous revisions and if you are not the kind of person who enjoys wading through pages and pages of technical specifications (few of us are) then there are perhaps three good books available which will make the whole process much more enjoyable. Even better, two of them are free downloads!
This book is actually not a book at all but a chapter-length article. It may be short but if you've have no previous experience with ebooks or this standard then going through this article with give you a great whirlwind tour of all the features along with what benefits the standard will bring to your own books.
Synopsis: This book discusses the exciting new format that is set to unleash a content revolution in the publishing world. Laden with features the printed page could never offer--such as embedded multimedia and scripted interactivity--EPUB3 will forever change what a book can be. This article walks you through the format and puts it in its place in the digital landscape, explaining why it is set to become the new global standard for ebooks as it also becomes the new accessible standard for ebooks.
Ready to take your books to the next level? Then I'd highly recommend you work through this book.
Written by some of the very people who created the standard itself, "EPUB 3 Best Practices" includes plenty of practical information, tips and tricks (and plenty of other gems) that will help you with all aspects of ebook creation; technology, content creation, and distribution.
Synopsis: This concise guide includes best practices and advice to help you navigate the format’s wide range of technologies and functionality. EPUB3 is set to turn electronic publishing on its head with rich multimedia reading experiences and scripted interactivity, but this specification can be daunting to learn. This book provides you with a solid foundation.
This is the final book in this series from O'Reilly. It's another free digital download with plenty of practical tips on making your ebooks much more accessible.
Synopsis: With the introduction of EPUB3, publishers now have the means to create a single rich data source for audiences of all reading abilities. Through practical tips and examples, Accessible EPUB 3 takes you inside the ebook format and explores how you can enrich and enhance content for all readers. Author Matt Garrish demystifies the process of making content easier to access, demonstrating how accessible practices are intertwined with standard content best practices.