What do you call a ball of 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 around itself to make for a smaller and more controllable yarn, which is easier to pack, and store. A hank also is a bit more manageable than a skein of yarn.

A ball of yarn would be wound from a skein or a hank.