Skein and ball are used interchangeably, so are skein and hank. A hank has to be wound into a ball to be used, but it's better not to do that until you're about to use it. A loaf type skein and a ball come that way from the store and you don't need to do anything else to them. Most patterns I've seen refer to skeins, but it may depend on the country the pattern comes from.
sue- knitting heretic