Depends on the organization. NPC goes by weight and classes are:
Men:
Bantam up to 143.25 lb
Lightwt up to 154.25 lb
Middlewt up to 176.25 lb
Light heavy up to 198.25 lb
Heavy up to 225.25 lb
Super heavy over 225.25 lb
Women BB have 4 classes, light, middle, light heavy and heavy, but not sure of the weight cutoffs. All this info is on http://www.npcnewsonline.com
You are weighed in before the prejudge.
NABBA goes by height. Classes are usually short, medium, med tall, and tall. http://www.nabba.com has the detailed info.