Because a skein is a method of wrapping yarn and not a measure of quantity, a skein can contain any length of yarn the manufacturer chooses. Most manufacturers measure the quantity by yards and use package labels to indicate how many yards the skein contains.
A skein of yarn is wrapped in an oblong shape that approximates that of an hour glass. According to About.com, the user can pull the yarn from inside or outside the skein. Users often rewrap skeins of yarn into balls to keep the yarn from tangling.
Some manufacturers wrap yarn into balls, and others wrap it into hanks. A hank is a loop of yarn, similar to a wrapped garden hose, that is twisted to prevent tangling.