Simply put, retreading is the process whereby selected and inspected worn tires, called "casings," receive a new tread.

Only sound, carefully inspected tire casings are used for retreading. The worn tread is buffed away and a new tread bonded to the tire body in a process very similar to the manufacture of a new tire. There are different processing techniques, but the ultimate objective is always the same - affixing a new tread through the application of heat, time and pressure.
Tire retreading is an established industry that began in the early 1900s and grew steadily. Today, there are approximately 850 retread plants throughout North America. These plants vary in size, from small operations producing 20 retreaded tires per day to the very large plants processing 1,000 or more retreads per day. Additionally, there are plants that retread only specialized tires, such as those for off-the-road, farm and construction equipment. Altogether, these plants retread millions of tires a year, using millions of pounds of synthetic and natural rubber. This represents over $3 billion in retread tires sold annually.

The above information is originate fromBandage.