When an item on Mackin.com indicates MP3, that means that it is an MP3 formatted CD instead of an electronic file or download. MP3 stands for Moving Picture Experts Group (MPEG), Audio Layer 3. The total file size is smaller, without sacrificing audio quality, allowing for more storage. While a regular audio CD holds 74 minutes of audio, MP3-CDs can hold up to 12.5 hours of audio. Also, if the primary use of the audiobook is to be played on a MP3 device, it is the most practical as the longer tracks create less file ‘clutter’ and MP3-CD audiobooks cost less.