Selective breeding is when farmers/ breeders choose specific traits for which to breed. This process often leads to very distinctive traits in different breeds/ varieties of the same species. If a member of the breeding stock does not conform to breed/ variety standards, then it likely will be "culled" from the breeding stock. In animal breeding registries are formed to protect the purity of the breed.
Cross breeding is the mating of two different breeds, strains, or varieties. The resulting offspring(s) are often referred to as a hybrid or crossbreed.