People specially in Mongolia and Tuva practice something, which We called Throat singing, or overtone singing. Term Throat singing is also used in inuit folklore, but this is not this same, which is practiced in Tuva, Mongolia and neighbor regions.
Overtone singing is style, when You sing two or three tones at once. It means, You sing overtones. Overtones are multiplications of one (fundamental) frequency, for example main frequency is 100 HZ and overtones are 200, 300, 400, 500 ETC.
Here is an example of mongolian overtone singing called Khoomei (Khöömei):