EPUB (short for electronic publication; sometimes styled ePub) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub - wikipedia

EPUB is designed for reflowable content, meaning that an EPUB reader can optimize text for a particular display device. EPUB also supports fixed-layout content. The format is intended as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale.

An EPUB file is a group of files that conform to the OPS/OPF standards and are wrapped in a ZIP file. The OCF specifies how to organize these files in the ZIP, and defines two additional files that must be included.

An example skeleton of an XHTML file for EPUB looks like this:

<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"> <head> <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=utf-8" /> <title>Pride and Prejudice</title> <link rel="stylesheet" href="css/main.css" type="text/css" /> </head> <body> ... </body> </html>

Open Packaging Format 2.0.1

The OPF specification's purpose is to define the various components of an OPS publication are tied together. This is accomplished by two XML files with the extensions .opf and .ncx.

The OPF file, traditionally named content.opf, houses the EPUB book's metadata, file manifest, and linear reading order.

The NCX file (Navigation Control file for XML), traditionally named toc.ncx, contains the hierarchical table of contents for the EPUB file - wikipedia