There are several kinds of bob haircuts, but in order to be a bob, the cut must be above the shoulders. Typically, a bob is cut at chin length, so that the hair curls under the chin, although a shorter bob is also possible.
Bobs come in many styles, including layered bobs, curly bobs, straight bobs and the asymmetrical bob. Each is slightly different, with the layered bob having high, short layers in the back, and the straight bob falling long, nearly to the shoulders, with little variation in the length of the hair. Like it's name, the asymmetrical bob has one side longer than the other, and curly bobs are cut to show off curly hair.