Format called EPUB or if you want electronic publication is free file type for electronic books. It is based on zip format, he has specific file structure. His main advantage is screen adjustment on device where is the file viewed. His only disadvantage is that he´s unofficial format, some people are criticizing him for unnecessary severity of unpacking e-books.
He was first released in 2007. That was due replacement of almost 10 years old format called Open e-book - .opf. This one was released in 1999. Latest version of EPUB was released on 11th October, 2011 and the version was 3.0. There´s a lot of advantages at this format. For example, there´s a good screen adjustment, it supports HTML5, audio and video. He´s also open file format, so it can be edited for free.
To open files for e-books you will need application called Cool Reader.