What is EPUB?
EPUB (also written as ePub, ePUB, epub, or EPub) is short for electronic publication. Files have the extension .epub. EPUB is an e-book standard that makes it possible to optimize text for display on different e-readers. Books in iBooks are published using the EPUB standard.
Why convert files or webpages to EPUB for reading on the iPad?
Unconverted files and webpages are static, meaning you cannot use the interactive features of iBooks when reading them on the iPad. Files and webpages that have been converted to EPUB format can be opened and read in iBooks; you can also use all of the features of iBooks, including dictionary, highlighting, etc.