What is yarn?
arange two sets of yarn is called yarn
Yarn comes in different "gatherings"--ways that it can be sold. Your question mentioned 2 out of the three: ball of yarn and skein of yarn. The third would be a Hank of yarn. A hank is a skein of yarn (which is yarn in a large oblong shape, and tied in a few places around the skein to maintain a bit of control over the yarn staying in that loose shape), which has been twisted…
It can vary greatly depending on your yarn tension and the size of the hook you are using. The more tension and the smaller the hook, the more yarn it will take. It also depends upon the type of yarn you are using. Thinner yarn (such as baby yarn or sport weight yarn) will require more yards to complete 1 square foot. Thicker yarn will require less yards to complete 1 square foot.