When you start knitting or crocheting, you discover a new vocabulary around yarn. Here is a visual summary of the different types of yarn windings. 
HANK: Loop of yarn, fastened into a continuous circle with ties.
FOLDED HANK: Hank folded in two with a label around it.
TWISTED HANK: Hank twisted into a braid and folded in two (also identified as a "skein")
SKEIN: Many people call the twisted hank a skein but it can also refer to a machine-produced ball.
BULLET SKEIN: Machine-produced ball in a bullet shape.
DONUT: Machine-produced ball in a donut shape.
BALL: Often hand-wound but can also be machine-produced.
CONE: Yarn winded on cone is stretched-thight and usually comes with a lot of yardage.
CAKE: Produced from winding hanked yarn onto a manual ball-winder
In preparation for this article, we did some digging, and several sources diverged on the terminology to use about yarn winding formats: there seems to be some confusion around "skein" and "hank."
Unfortunately, we have not found any information on the Craft Yarn Council website to settle the question but we found this very interesting article on Interweave website. So if you think the information presented here needs to be corrected, please speak up! 😃