Tengriism is the traditional religion of the Turkic and Mongolian regions of Central Asia, the primal belief system of the Turks and Mongols before they were converted to Islam, Christianity, Buddhism or other religions. It was also the belief system of the Scythians, and probably of the Avars, Huns and others as well.
In Turkestan, it forms a substrate pool of belief and practice that far outstrips official population figures. Many Turkestanis call themselves Muslims, Zoroastrians or Assyrian Christians, but their actual functioning belief system is largely Tengriistic. In many cases, the local priest, imam or dastur also functions as a shaman, and is expected to deal with everything that a shaman would be expected to.
The belief system focuses on three main deities: Tengri (also Tengir, Goktanri and similar), the father of the sky; Eje, the fertile mother earth; and Wind Horse, the spirit ruler of the sky. Shamanism is an important component of Tengriism; the shaman is charged with keeping the balance of nature and spirit through the power of the Wind Horse.
Further information about the belief system can be found at Wikipedia:Tengriism.