what is epub extension?

"EPUB is an e-book file format with the extension .epub that can be downloaded and read on devices like smartphones, tablets, computers, or e-readers. It is a technical standard published by the International Digital Publishing Forum (IDPF). The term is short for electronic publication and is sometimes styled ePub" https://en.wikipedia.org/wiki/EPUB. 

Here some software to open epub;
1. Epub File Reader, it is free software it is nice and easy to use. 
    u can download at http://www.epubfilereader.com/

2. Sumatra PDF is a free and open-source document viewer, it supports many      document formats including: Portable Document Format (PDF), Microsoft            Compiled.HTML. Help (CHM), DjVu, EPUB, FictionBook (FB2), MOBI, PRC, Open XML Paper Specification (OpenXPS, OXPS, XPS), Comic Book Archive file (CB7, CBR, CBT, CBZ).It is developed exclusively for Microsoft Windows, but it can run under Linux with help of WINE. download the sumatra pdf here