Yarn weight is the thickness of a strand of yarn, not its physical mass. Standard categories run from lace (thinnest) through fingering, sport, DK, worsted, aran, bulky, and super bulky (thickest), each suited to different projects and gauges.
Matching yarn weight to a pattern is essential — substituting a heavier or lighter yarn changes the gauge and the finished size. The Craft Yarn Council numbers weights 0–7 to make substitution easier.