High speed multifunction port (interface) designed by Intel and first manufactured on a shipping product by Apple. Thunderbolt was developed under the name Light Peak and combines PCI Express with Display Port over one serial connection.
Although Thunderbolt and Light Peak were designed to operate over fiber connections, the initial implementation is over copper connectors that are capable of delivering 10W of power as well as four data lanes.
Thunderbolt is a natural progression for Mac users since it shares many similar features with the older Firewire interface such as being hot pluggable, very high speed compared to peer interface specifications at introduction, provides more power than competing specifications and working in target mode direct access to the internal drives of Mac computers without requiring the OS to run.
Thunderbolt differs from all other common computer interfaces currently in production as it uses active cables with embedded chips to boost the performance and control noise on the cable. This allows for thinner and/or longer cables without degrading the transmission speed over the active cable run.
If a Display Port device is connected to the thunderbolt port on a Mac, the 4 lanes output 4 x 5.4 Gbit/s for an aggregate delivery of 21.2 Gbit/s. When connected to another Thunderbolt device, the lanes reconfigure to supply a pair of 10 GBit/s bidirectional PCI Express data connections with each connection having one lane for input and the other for output.
Much more technical detail as well as industry wide references is available at the wikipedia entry for Thunderbolt.