epub is a an open e-book file format standard.
“Epub” format might be compared to “.pdf” format, in that it is designed to contain the contents of a book or document intended to be read or viewed. Epub format is optimized specifically for ebook reading applications or devices.
Most all ebook reading devices either support epub format, or use it as their primary ebook distribution and display format. The notable exception is the Amazon Kindle, which does not support epub format.
There is an assortment of applications available for most any other platform that can be used to read books distributed in epub format.
Further reading: EPUB on Wikipedia.