"What's a Yak????"
Okay, good, this is a good place to start. A yak, most simply, is a hairy cow-like animal. It's a mammal, and... well, they're hairy... no, this isn't working. Not at all. I think in this particuar case, a visual demonstration would work better...
All of the Following are NOT YAKS!
CAT
DOG
ALIEN
THESE are yaks!!! (See, I told you they were hairy!!!)