In Short

Thunderbolt is a high-speed hardware interface built into all current Mac models. It replaces FireWire as the high-speed peripheral interconnect of choice for the Macintosh, and can be used for everything from external hard drives to network connections to driving external displays.

Thunderbolt uses a trapezoidal interface that's the same as mini DisplayPort. In fact, Thunderbolt combines PCI Express and DisplayPort into one single serial connection alongside electric power, all transmitted over a single cable.

Originally code-named Light Peak, Thunderbolt was develop by Apple and Intel and was first shown off during an Intel event in 2009, running on a modified Mac Pro. Apple introduced it as Thunderbolt with the 2011 MacBook Pro model, and it's made its way out to every other Mac model since then. Thunderbolt is now commonly available in one of two forms: The original Thunderbolt interface, rated at 10 gigabits per second transfer speed, and Thunderbolt 2, rated at 20 gigabits per second. A new 40 gigabit per second implementation will come when Intel releases its Skylake processor, expected in 2015.

Thunderbolt 2 was introduced on new Mac models starting in 2013, with the 2013 MacBook Pro with Retina display. It was also added to the Mac Pro.