via AKC-REUNITE
 A microchip contains a unique ID number, specific to your pet. It is about the size of a grain of rice and is implanted underneath the skin between your dog or cat’s shoulder blades. Implantation is quick and virtually painless, like a normal pet vaccine. Your pet’s microchip contains a unique number that you then enroll with AKC Reunite so we can contact you and your alternate contacts when your pet is found.
 
It is important to clarify a microchip is NOT a GPS or tracking device because it has no power source! Instead, a microchip contains a small antenna that can only be read by a microchip scanner used by veterinarians, shelters, and animal control. When the scanner passes over the microchip, it reads the microchip ID number. It is not exactly the same, but similar to scanning a barcoded item at the grocery store.