Sorry - I forgot the second part of your question.
"Worsted" has two meanings.
- One refers to the way the yarn is spun: worsted yarn is spun of long fibers that lie parallel to each other and produces a firm, smooth, strong yarn. ("Woolen" yarn, on the other hand, is fuzzier and weaker, but very warm.)
- "Worsted" is also the an American term for medium-weight yarn. It's thinner than bulky weight yarn and thicker than DK.
Hope this isn't too confusing.