There are two major kennel clubs in the U. S. They are the American Kennel Club (AKC) and the United Kennel Club (UKC).
The AKC is the oldest, largest, best-known (thanks to the Westminster Dog Show) and most rigid.
The UKC is second oldest, second largest and generally considered more accessible to ordinary dog owners.
Both register dogs and both have standards of conduct for breeders.
Generally, however, the AKC is more concerned with breeding dogs that meet the physical demands of a breed standard.
Sometimes that, in my opinion, has led to unfortunate side effects such as miniaturizing dogs to the point they developed slipped kneecaps and collapsed wind pipes.
The UKC, on the other hand, stresses the whole dog - both looks and temperament. It is well known for its performance events: Coonhound field trials, water races, agility trials and hunting tests for the retriever breeds.
Your own interests would determine which kennel club would appeal to you although you register in both.
There are other kennel clubs and other “purebred" registries. Be aware that some of these exist only because the breeders can't meet the standards of the AKC or the UKC.
For example, one of the largest alternative kennel clubs was started by a group of Missouri dog breeders who were booted out of the AKC because they violated safety and health standards.
When they could no longer advertise their puppies as being AKC-registered, they created a registry of their own.
Another registry was started to handle a franchised chain of pet shops.
It's easy to tell a true registry from a quick-buck operation. The real kennel clubs require a pedigree and often DNA or genetic testing.
The quick-buck ones require you to send a check. Many do not require your dog to be a purebred and one of them even registers wolf/dog crossbreeds.
Many reputable breeders are registered with their breed club as well as the AKC or UKC.
Many breed clubs have a stricter code of ethics than either the AKC or UKC. It's reassuring to only deal with a breeder who dual-registers or only registers with the breed club named in the AKC's list of breed clubs.
I don't know why you would ever want to be registered with any kennel club other than UKC or AKC.
Louise Louis is a certified canine specialist and creator of the popular website on small dogs, http://www.ToyBreeds.com