Yarn balls are sold in different shapes, some are ready to use, while others are twisted skeins that need to be wound beforehand.
Ball: This is a common type of yarn ball, you have access to only one end of the yarn tail. Sometimes you can pull the inside tail out of the center, but it’s not recommended to start with inside tail because you can get stuck in a mess.
Center pull ball: A center pull ball of yarn is wound and ready to use, it allows you to access both ends of the yarn tail, you can either remove the label and find the outside tail or pull the inside tail out of the center to start with. A center pull ball won’t roll around while you’re knitting.
Cake: A cake is a type of center pull ball, it is flat on the top and bottom and is created by a ball winder.
Hank (or twisted hank): Yarn is loosely wound into a ring shape and then twisted on itself to make a hank, a hank needs to be wound into a ball before using.
Cone: Coned yarns are often used in knitting machines.