The breeder will probably request a contract be signed, stating that the puppy will be spayed or neutered when it reaches the appropriate age. REMEMBER, the responsible breeder only wants the best possible specimens of the breed to contribute to future generations, so you should be willing to accept this agreement if it applies to your puppy. Spaying or neutering your dog also decreases the risk of some future health problems.
Contracts also often cover health conditions that may arise in the puppy over time (and if one develops if there is any refund/replacement), or "First Right of Refusal" where the breeder is able to get the puppy back should the owner no longer be able to care for (or want) it.
Contracts are often written as "co-ownership" agreements. This allows the breeder to ensure it is not bred without permission from the breeder, since the breeders signature may be required for the AKC papers. Sometimes puppies are co-owned so the breeder can also show the puppy in the "Bred By" class of conformation shows.