The computer memory in the microchip contains a unique 15 digit number - no two dogs will ever get the same number.
Apart from the number the chip contains no other information.
A radio signal transmitted from a scanner is used to read this number through the skin of your pet.
In addition to the number, the microchip generates a reliability check to guarantee that your pet's identifying number is read accurately. This all takes place in less than a second.
After the scan the vet consults a database to find a matching number and thereby obtain the owner's contact details.