A way to 'tag' cats so that if lost they can be reunited with their owners. The process involves injecting a microchip under the skin, usually between the shoulder blades. The cat's owner and their contact details are then registered with a centrally held database along with details of the code. Rescue Centers and vets usually keep the scanners that allow the microchip details to be read: the device is simply passed over the cat and the code appears on screen. The advantage of microchipping is that it is a permanent form of identification, unlike collars and tags that may be lost or result in injury to the animal.

SOURCE: cats.org.uk