There are many file formats used for eBooks. Usually, but not always, the file extension matches the name of the format. In some cases the extension may have two forms, where one form is limited to 3 characters (consistent with early Windows requirements).

Main Formats

• AZW - An Amazon proprietary format. This is usually the MOBI format with or without DRM. The DRM is unique to the Amazon Kindle. Files with this extension can be any of the Kindle formats.
• AZW1 - An Amazon proprietary format. It is the TPZ format always with a custom DRM.
• AZW3 - See KF8.
• AZW4 - An Amazon proprietary format. It is the PDF format in a PDB wrapper, and usually (always?) with DRM.
• EPUB - An open format defined by the Open eBook Forum of the International Digital Publishing Forum. It is based on XHTML, XML and CSS2. It is an evolving standard. Current specifications are found at the idpf web site. Adobe, Barnes & Noble and Apple all have their own (incompatible) DRM systems for this format. There is now a new version of this format called ePub 3 but it is not yet in wide use.
• KF8 - (Also called AZW3) It is basically ePub compiled with the PDB wrapper and with Amazon DRM. This format is supported by all Amazon readers from the Kindle Keyboard 3 onwards.
• KFX - A semi-compiled format from Amazon designed to give better typography on Kindle devices, comes with a new DRM system.
• MOBI - MobiPocket format, usable with MobiPocket's own reading software on almost any PDA and Smartphones. Mobipocket's Windows PC software can convert .chm, .doc, .html, .ocf, .pdf, .rtf, and .txt files to this format. Kindle uses this format, as well.
• PDB - Palm Database File. Can hold several different e-book formats targeting Palm-enabled devices, commonly used for PalmDOC (AportisDoc) e-books and eReader formats as well and many others.
• PDF - Portable Document Format created by Adobe for their Acrobat products. It is the defacto standard for document interchange. Software support exists for almost every computer platform and handheld device. Some devices have problems with PDF since most content available is scaled for either A4 or letter format, both of which are not easily readable when reduced to fit on small screens. Some Readers can reflow some PDF documents, including the Sony PRS505, to accommodate the small screen. Some eBook readers, including the iRex iLiad, have a pan-and-zoom feature that aids readability, but extracts a price in ergonomics.
• PRC - Palm Resource File. Often holds a Mobipocket eBook but occasionally holds an eReader or AportisDoc eBook.
• TPZ - Topaz file extension used on Amazon Kindle. Topaz is a collection of glyphs arrange on pages, along with an unproofed OCR text version. An Amazon proprietary format, used to make older books available quickly, since conversion is essentially automatic from scans of the pages of a book, but it reflows very well.