A dzo is the result of breeding a yak with a domesticated cow. Male dzo are infertile, but female dzo are capable of breeding. Female dzo are sometimes referred to as dzomo. The average work life of the dzo is 14 years, compared to 10 years for a standard yak. Dzo are very sure-footed and are often used to transport heavy loads or plow fields. Most of the world's dzo are found in or around Tibet and China.
According to www.bagelhole.org, which for some reason has a long article on yaks and dzo, some farmers tie shut the mouths of dzo and yaks. They believe that if yaks and dzo drink water after heavy labor, they will form tumors in their necks.
I have infrequently seen the term dzo used to refer to a standard yak.