Originally marketed under the name Light Peak, Thunderbolt is an interface that is used to connect peripheral devices to a computer. It is a serial interface that combines existing PCI Express and DisplayPort technologies. It allows for long distance data transfer over cheaper cables capable of carrying DC power.
The first two iterations of Thunderbolt can transfer data at a rate of up to 20 Gbit/s and utilize the same connector you can find on a Mini DisplayPort cable. The third version can transfer data at a rate of up to 40 Gbit/s and utilizes a USB Type-C connector.
What are the cables made out of?
There are two types of Thunderbolt cables, one which utilizes copper wiring and the other utilizing optical wiring. Although Thunderbolt cables were originally intended to use fiber optic cables, those versions have had limited releases. Copper wiring is much cheaper and allows the cables to supply power, which is why it was used almost exclusively. Eventually, Intel intends to combine both copper and optical wiring for the faster bandwidth speeds of optical, while utilizing the power of copper wiring.