Selective breeding allows the breeder to select traits which they find desirable and mate their stock so that the trait becomes more common with each generation. A side effect of selective breeding is that often undesirable traits are overlooked when pursuing only one specific characteristic.
Breeders have been engaging in selective breeding for hundreds of years. The practice is common in agriculture, animal husbandry and even in dog breeding. Dogs have been bred for specific traits for generations. The reason that there are so many distinct breeds of dogs is that they have been selectively bred for certain characteristics. For example the German Shepherd was bred for service as a military and police dog. The breeder selected dogs based on their physical characteristics and personality. Things that were deemed desirable were bravery, size, speed and endurance. This breed of dog is used extensively in police and military work even today.
The downside to selective breeding is that any time only specific characteristics are valued, there is the possibility that undesirable characteristics can find their way into the breed. The German Shepherd, while brave and strong, has a genetic pre disposition to hip dysplasia. Other examples of negative impacts of selective breeding in dogs include miniature poodles' tendencies to experience seizures or breathing problems in English Bulldogs.