Classes in USA:

Puppy - For dogs between six and twelve months of age, that are not yet champions

Twelve-To-Eighteen Months - For dogs twelve to eighteen months of age, that are not yet champions

Novice - For dogs six months of age and over, which have not, prior to the date of closing of entries, won three first prizes in the Novice Class, a first prize in Bred-by-Exhibitor, American-bred, or Open Classes, nor one or more points toward their championship
Amateur-Owner-Handler For dogs that are at least six months of age that are not champions.  Dogs must be handles in the class by the registered owner of the dog and is limited to exhibitors who have not, at any point in time, been a professional dog handler, AKC approved conformation judge, or employed as an assistant to a professional handler (effective January 1, 2009)

Bred By Exhibitor - For dogs that are exhibited by their owner and breeder, that are not yet champions

American-Bred - For dogs born in the United States from a mating which took place in the United States, that are not yet champions (mandatory class).

Open - For any dog of the breed, at least 6 months of age (mandatory class).
All the dogs that won first place in a class compete again to see who is the best of the winning dogs.
Males and females are judged separately.
The Winners Dog and Winners Bitch compete with the champions for the Best of Breed award.

For more information go to: http://www.akc.org/