Animal shelters are set up by local governments or private individuals to provide a temporary home for lost or abandoned animals. Unfortunately, resources are seldom adequate to support a large number of animals, so animals that are not adopted right away are euthanized.
A small number of shelters are "no-kill" shelters, which support their charges for the remainder of their lives. However, funding is limited, so the number of animals that can be taken in is relatively low.
Some people consider pet stores a good place to get a new pet, and a shelter a good place to leave the pet when it gets sick or is no longer wanted. Individuals and organizations are attempting to change that point of view by educating owners and potential owners about the large number of adoptable animals available from shelters, and the limited resources available to keep these animals alive. They promote the idea that a shelter or reputable breeder are the best places to get a new pet, and that anyone who isn't ready to give their furry friend a "forever home" shouldn't get a pet in the first place.