What are the Three Turnings of the Wheel of Dharma according to Tibetan Buddhism?

Tibetan Buddhists often talk about the Three Turnings of the Wheel of Dharma... What are the Three Turnings of the Wheel of Dharma according to Tibetan Buddhism? Where did they take place according to Tibetan Buddhists? How do Tibetan Buddhists interpret these Three Turnings? Is there disagreement within Tibetan Buddhists schools about which teachings are definitive and which are provisional? Do Tibetan Buddhists think these Three Turnings are contradictory?