This page explains the three types of e-book file formats available for purchase on this site. EPUB, PDF, and MOBI are the three main file formats, and they will cover the large majority of ways you can read an e-book. (Most of this information is taken from Wikipedia, in June of 2012.)
If you are new to ebooks, EPUBs are highly recommended; there are multiple free EPUB-reading apps and softwares. The EPUB will give you a more enjoyable reading experience than buying the PDF will.
The EPUB format is rapidly gaining popularity and as of 2011 is the most widely supported vendor-independent XML-based e-book format. If you are reading on a computer, there are a number of free, downloadable programs you can get to read EPUBs, including Adobe Digital Editions, Caliber, and Stanza. Ereaders and tablets that can read epubs include Kobo eReader, Blackberry Playbook, Apple’s iBooks app running on iOS devices such as the iPhone, iPod Touch and iPad, Barnes and Noble Nook, Sony Reader, BeBook, Bookeen Cybook Gen3, COOL-ER, Adobe Digital Editions, Lexcycle Stanza, BookGlutton, AZARDI, FBReader, Aldiko, Moon+ Reader and WordPlayer on Android, Freda on Windows Mobile and Windows Phone 7, the Mozilla Firefox add-on EPUBReader, and Okular. Several other desktop reader software programs are currently implementing support for the format, such as dotReader, Mobipocket, and uBook. EPUB CANNOT be read by the Kindle.
PDFs are widely supported, with almost all computer systems having either some native ability to read them, or with free downloadable software to read them. PDFs are also supported on many e-book readers including: Mobipocket, iRex iLiad, iRex DR1000, Sony Reader, Bookeen Cybook, Foxit eSlick, Amazon Kindle, Barnes & Noble Nook, the iPad, PocketBook Reader, Bebook Neo and the Kobo eReader. On iPad, there are also many readers such as Stanza, iBook, and FlipReader.
The most important thing to know about PDFs, though, is that they are not “reflowable,” meaning their layout stays the same no matter what screen size or resolution you view them at. (Although newer PDFs can now be made to be reflowable, Reading Poker Tells PDF is not.) This is different than other ebook files which reflow the text based on the display, sort of like websites do. This makes PDFs very unwieldy for viewing on smaller devices.
This is the format for the Kindle.