The microchips put in most pets are like Jane said, just for identification, not tracking.
They are a small passive "chip" about the size of a grain of rice that has a unique number encoded in it. There is no power supply to it, and it only responds when queried by the proper device. They are "injected" by a large syringe, and do not normally require surgery to implant.
When a device called a transponder is passed over it, it can read that number and it will thereby identify your pet by means of that number.
There are small tracking devices. However, they are usually used by researchers, and occasionally other folks, like falconers, to track animals that may travel long distances. These devices emit an active signal that can be read by the proper receiver. ...but you still have to be within a certain range of them (although some can be tracked by satellites).
These are limited to the battery life of he device, and are VERY expensive by comparison. They would also require surgery to implant.
If you get your animal "micro-chipped", if they ever end up at an
animal shelter, the shelter will check for he chip and be able to find you. I think that is what you are talking about.
I hope that's the explanation you were looking for..
Bob