An M4B file is similar to a .M4A file, but it is saved as an audiobook instead of a standard music file. This means it can include bookmarks and other features supported by audiobook players. The M4B format is used by the iTunes Store as the standard format for distributing downloadable audiobooks.
About Apple iTunes
iTunes is a media player developed by Apple. It was originally designed as an audio player, but now has many other features. iTunes supports audiobook and video playback and allows users to access the iTunes Store, which contains downloadable content. It is also the standard program used for syncing data with iPods, iPads, and iPhones.