The EPUB file format is based in the XML programming language. XML is a fairly widespread language used to transport many kinds of data. Consequently, EPUB books are compatible with a number of devices and programs. You can open EPUB books in software such as Adobe Digital Editions, Calibre and Sony Reader. The books available in the NOOK Book Store are in the EPUB format.
The NOOK Simple Touch and the NOOK tablet models can also read PDF files. However, PDF files come in many different shapes and sizes; some files consist entirely of images or text while others are a combination of the two media. Consequently, your ability to zoom, highlight and manipulate PDFs may be different from your experience with EPUB books.
Digital rights management is a way of linking purchased digital media with the purchaser. If you are viewing a book with DRM on your NOOK or NOOK app, you may be asked to input some of your account information. The purpose of this process is to confirm that you are an authorized user of the media. DRM is also involved when lending your e-books to another person; if you lend your e-book to somebody else, you surrender your DRM authorization until the lending period is over. Because of DRM, you may not be able to access some NOOK books even though they are in the EPUB format.
If you do not own a NOOK device, you can still view NOOK e-books using a NOOK app. The NOOK apps for Android, iOS, Windows and Mac OS X all support reading NOOK Book Store books. Furthermore, you can sideload other e-books through the NOOK app. In this case, sideloading means that you can transfer other EPUB books to your device and open them on the NOOK app. If those documents do not have DRM, you should be able to open them regardless of whether they came from the NOOK Book Store.