Any book produced in an digital format is an ebook. Ebooks may be created in EPUB, Microsoft Word, PDF, text, MOBI or a wide variety of other formats.
Due to the book-centric nature of the term, the name is almost never used in standards development. "Ebook" formats are often just as capable of being used for journals, articles and magazines, but the name detracts from these uses. The term digital publication is more commonly used to refer any type of publication in a digital format.
EPUB is a distribution and interchange format for digital publications. It provides a means of representing, packaging and encoding structured and semantically enhanced Web content — including HTML, CSS, SVG and other resources — for distribution in a single-file container.
EPUB was originally developed by the IDPF, but is now maintained by the EPUB 3 Community Group in the W3C
- EPUB 2
EPUB 2 is the first version of EPUB introduced in 2007.
EPUB 2 uses XHTML 1.1 as the markup standard for content, but notably omits support for the modules that provide interactive and server-side functionality (i.e., scripting, forms, events and server-side maps).
EPUB 2 was quickly succeeded by EPUB 3 due to its many limitations (poor styling support, limited accessibility features, rudimentary support for international languages, etc.).
A format called the Open Ebook Publication Structure was the predecessor to EPUB 2 and where the "2" comes from.
Refer to the EPUB 2 specification for more information.
- EPUB 3
EPUB 3 is the current version of EPUB. It succeeded EPUB 2 in 2011 and moved the format to XHTML5 and SVG for representing content.
EPUB 3 is also the successor to the DAISY 3 format as it provides improved accessibility in a format the publishing industry already supports.
Refer to the EPUB 3 specification for more information.
- EPUB Container
- EPUB Content Document
Although an EPUB publication appears like a single document when read, the content is typically broken up into many separate documents inside the container. These documents are referred to as content documents and come in one of two formats: XHTML5 and SVG.
A feature of the package document called the spine defines the order to present the content documents in, giving the illusion of a single seamless document.
The EPUB navigation document is a special document in an EPUB publication that provides the main navigation aids (e.g., the table of contents, page list and landmarks).
The document is primarily intended to for use by reading systems. When a user clicks a button to open a table of contents, for example, the information in that table of contents has been read from the navigation document.
It is possible to reuse the EPUB navigation document in the content of an EPUB publication, but in this case the document appears like any other HTML document.
- EPUB Publication
- EPUB Style Sheet
EPUB 3 intially defined a baseline set of CSS that authors were encouraged to use and reading systems to support under the name of "EPUB Style Sheets".
This naming caused confusion, however, as it was sometimes misunderstood that EPUB was defining its own version of CSS. As a result, the name is no longer used in the EPUB specifications and has been replaced by more general references to CSS.
Any reference to "EPUB Style Sheet" should be read as synonymous with "CSS Style Sheet".
epub:typeis the name of an attribute introduced in EPUB 3 to provide semantic inflection (the ability to add additional information about the purpose of elements).
It is used with the EPUB Structural Semantics Vocabulary to enhance the markup in EPUB content documents.
Although initially hoped that the attribute would provide a bridge between publisher in-house semantic markup needs and accessibility, the attribute never gained traction for accessibility.
Epubcheck is a validation tool that inspects EPUB files for problems. Among other things, it reports fatal errors that can prevent a file from opening, general errors that can lead to the content not working as expected, and it also issues warnings about practices that are not recommended.
For more information about epubcheck, refer to the Epubcheck wiki.
Escapability refers to the the ability for non-visual readers to leave the current structure that is being read and continue reading at the next logical spot.
A table is an example of an escapable structure, as assistive technologies provide the ability "escape" from the current table cell and move to another (or even to exit the table entirely and continue reading from what follows).
Lists are another example of escapable structures, as users of assistive technologies are no more interested in reading every list item in some lists than any other reader.