[targonrotts]

in a typical akc conformation show you have the following classes:
6-9 puppies
9-12 puppies
12-18 dogs
novice
bred by
am bred
open
some clubs/shows do not offer novice class...eventually it might be phased off and possibly replaced with an amateur handler class. for more info on that see American Kennel Club - akc.org and see rules and regulations applying to dog shows.
puppy classes are considered 6-9 and 9-12. 12-18 is considered an adult class(when calculating fees)only 6-9 and 9-12 is eligible for a best puppy award if offered.
novice, when offered, (and rarely used),is for dogs never won a first place in another class or no more than 3 firsts from novice.(if rules have not changed from when i read them last).
bred by-you must be owner/breeder/handler, over 6 months of age, and not yet a champion.
am bred-born in the usa, over 6 months of age
open-born anywhere, over 6 months of age.

of course, typically you do not see a puppy in am bred or open.

different handlers/owners have different opinion/reason why they would enter open vs am bred.