It happens everywhereIn every community, in every state, there are homeless animals. In the U.S., there are an estimated 6-8 million homeless animals entering animal shelters every year. About half of these animals are adopted, and tragically, the other half are euthanized. These are healthy, sweet pets who would have made great companions.
The number of homeless animals varies by state—in some states there are as many as 300,000 homeless animals euthanized in animal shelters every year. These are not the offspring of homeless "street" animals—these are the puppies and kittens of cherished family pets and even purebreds.

The single most important thing that we can do to save cats and dogs from all the suffering and death that their overpopulation causes is to spay and neuter them. Spaying and neutering are routine, affordable surgeries that can prevent thousands of animals from being born, only to suffer and struggle to survive on the streets, be abused by cruel or neglectful people, or be euthanized in animal shelters for lack of a loving home. 

Sterilized animals live longer, happier lives. Spaying eliminates the stress and discomfort that females endure during heat periods, eliminates the risk of uterine cancer, and greatly reduces the risk of mammary cancer. Neutering makes males far less likely to roam or fight, prevents testicular cancer, and reduces the risk of prostate cancer. Altered animals are less likely to contract deadly, contagious diseases, such as feline AIDS and feline leukemia, that are spread through bodily fluids.

What is Spaying and Neutering?
The surgical procedure, performed by a veterinarian, renders the animal incapable of reproducing.  Female animals get spayed, males get neutered. 
During surgical altering, a veterinarian removes certain reproductive organs. If your cat or dog is a female, the veterinarian will usually remove her ovaries, fallopian tubes, and uterus. If your pet is a male, the testicles are removed.