A hank and a skein are just two different ways of wrapping a ball of yarn. As long as you know the yardage you will be fine. Most of the usual yarn that you would buy at Michael's or Wal-Mart and places like that are wound into skeins. Not any different (as far as I'm concerned) than a regular old ball of yarn. A hank is something like what's pictured here
Hope that helps.