What do Jumbo, Extra Large, etc. Egg Sizes Mean?

Eggs are categorized into classes based on their weight per dozen. 

Size or weight class	Minimum net weight per dozen (ounces)	Minimum net weight per egg (ounces)
Jumbo	30	2.5
Extra Large	27	2.25
Large	24	2
Medium	21	1.75
Small	18	1.5
Peewee	15	1.25