Introduction to Apple's M4b Audiobook Format
Files ending with the .M4b extension can be identified as audiobooks -- these are normally purchased from Apple's iTunes Store. They are similar (but not identical) to files ending in the .M4a extension which also use the MPEG-4 Part 14 container format (commonly referred to as just MP4). The MP4 format is a metafile wrapper which can hold any type of data (both video and audio) and acts as a container for M4b audio streams. Incidentally, the MP4 container format is based on Apple's QuickTime platform but it differs slightly by having extended MPEG features and Initial Object Descriptor (IOD) support -- this complex sounding jargon just means elements to access MPEG-4 content.
The audio in a M4b file is encoded with the AAC compression format and can therefore be protected with Apple's FairPlay DRM copy protection system in order to restrict access to only computers and iOS devices that have been authorized via iTunes.
Advantages of the M4b Format for Audiobooks
The main advantage of listening to M4b audiobooks is that unlike MP3, WMA, and other commonly used audio formats, you can bookmark a recording at any point. If for example you are listening to a book on your iPod or iPhone that you have purchased from the iTunes Store, you can conveniently pause (bookmark) it and resume where you left off at another time. This is a lot more convenient than having to skip through the whole book trying to find the exact point you got to. Audiobooks can be a few hours long and so the M4b format is the perfect choice due to its bookmarking feature.
Another advantage of the M4b format is it enables a large audiobook to be split up into chapters just like a physical book. Using chapter markers, a single M4b file can be segmented into manageable chunks for the listener to use just like the chapters of a book.