A pet microchip is a tiny computer chip housed in a type of glass made to be compatible with living tissue. It's about as long as a grain of rice, but slightly thinner. The microchip is implanted under the skin with a needle and special syringe. The process is similar to getting a shot.
The chip has a random number on it that can be read with a scanner. This random number links to a file at the microchip company that contains your personal information. Most shelters, Animal Control Officers, and vets scan pets for chips to help them get back home.
Many people confuse microchips with tracking devices. But microchips are more like internal i.d. tags.