It is called polyphonic singing aka overtone chanting, harmonic singing, or throat singing. Bjork does some (but NOT all) of her singing in a polyphonic manner.
Also refer to Wikipedia documentation as follows:
Overtone singing – also known as overtone chanting, harmonic singing, or throat singing – is a type of singing in which the singer manipulates the resonances (or formants) created as air travels from the lungs, past the vocal folds, and out of the lips to produce a melody.
The harmonics (fundamental and overtones) of a sound wave made by the human voice can be selectively amplified by changing the shape of the resonant cavities of the mouth, larynx, and pharynx. This resonant tuning allows singers to create apparently more than one pitch at the same time (the fundamental and a selected overtone), while actually generating only a single fundamental frequency with their vocal folds.
Each note is like a rainbow of sound. When you shoot a light beam through a prism, you get a rainbow. You think of a rainbow of sounds when you sing one note. If you can use your throat as a prism, you can expose the rainbow – through positioning the throat in a certain physical way, which will reveal the harmonic series note by note.