The concept: an album of music created almost solely using the human voice.
This album could be described as "off the wall", however I believe this is due to the concept in question - had Björk created the same songs using instruments, it would not be as huge a departure from her previous albums. This is not a good album to use as an introduction to Björk.
It's hard to see how she'll perform some of the tracks live, e.g. Öll Birtan, which is done completely using multiple Björk voices. Thus far my favourite tracks are Who Is It, Mouth's Cradle, and Triumph of a Heart (Weirdest. Björk. Track. Ever).