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.m4b is 100 percent identical to .m4a format, but just with different name. So, .m4b is, like .m4a, an audio file wrapped within MPEG-4 container format. The audio encoding can be virtually anything, but most collomly used encoding for both, .m4a and .m4b, files is the AAC.
The only reason for the duplicate naming convention is so that the Apple's iTunes software and iPod players can recognize the file as an audio book rather than a normal audio track and thus allow "bookmarking" the file.