What is worsted weight yarn_

worsted is a weight measurement; it's a mid-weight yarn that has multiple plies (threads) twisted together (I think it's traditionally four-ply???).

Here's the standards chart of different kinds of yarn
yarn standards: weight (http://www.yarnstandards.com/weight.html)

Maybe someone can be more descriptive then me but I think the best thing to do when learning about yarns is to go into LYS and touch some different weight yarns

ETA: what Ingrid said. She's so smart! (but apparently I can't type edit #47)

Worsted weight is the most common weight yarn--it has to do with the thickness and can be any number of plies. It usually knits up to 4-5 stitches per inch, depending on the needle size.