For popular yarn brands, yarn weights are standardized to make crafters lives easier. Never again guess if that unlabeled yarn at the bottom of your stash is right for your gauge sensitive pattern!!!
CYC (Craft Yarn Council) Yarn Weight Categories
WPI stands for Wraps Per Inch. This is a method to determine yarn weight. This means that if you take something like a ruler, and wrap the yarn with the strand wraps side by side, the number of wraps will tell you the weight of the yarn.
Yarns with 6 wraps or less are considered super bulky, like roving.
Yarns with 7 or 8 wraps are considered bulky, like chunky.
Yarns between 9 and 11 wraps are considered medium, like worsted.
Yarns between 12 and 14 wraps are considered light, like DK.
Yarns between 15 and 18 wraps are considered fine, like sport.
Yarns with 19 or more wraps are considered super fine like sock yarn.
More about the CYC can be found at http://www.craftyarncouncil.com/ .