Yaks diverged from cattle millions of years ago. There is some suggestion that they may be closely related to Bison.
Thousands of years ago yaks were domesticated primarily for their milk, fiber, and meat. Their dried poop is an important fuel in Tibet, and often is the only fuel available on the high treeless plateaus.
Being extremely well suited to the terrain and climate of the region, yaks have been used historically as a reliable method to transport food and supplies in and out of the Himalayas. They are famous for accompanying climbers in these areas. Yaks don’t eat grain (which could be carried on long journeys). They will starve unless brought to a place where there is grass.
The strength and endurance of a yak make it a useful animal for farmers. Their milk can be made into a cheese called chhurpi. (Byaslag in Mongolian.) Butter made of yak's milk is used in butter tea; as fuel for lamps; and to create sculptures for religious festivities.
Yak racing, skiing, and polo have been popular events in Mongolian horse festivals.
One way that yaks differ from cattle is that they don’t moo. They grunt. Contrary to popular belief they don’t stink; they have almost no odor at all.
Sometimes yaks are crossbred with cattle which produce infertile males (called a dzo), or fertile females (known as dzomo), which may be crossed again with cattle. Yaks have also been bred with Bison, Gaur (Indian Bison), and Banteng cattle.