FireWire, also known as IEEE 1394 or i.Link, is type of port built into many computers and video recorders. It allows the high speed transfer of data between two devices. It is most often used in A/V applications. Most camcorders come with the interface built in. It is becoming more common for personal computers and specifically laptops to have the port built in.
Apple coined the term FireWire for the interface. The name have originated from the fact that it is used in high performance fire alarm systems. Apple has driven and funded most of the development for the standard.
FireWire competes mostly with USB 2.0 which has a much broader market. The original iPods had FireWire ports, which were later dropped in favor of USB 2.0. USB 2.0 has a larger proportion of the market and is cheaper to include in devices.
Almost all major operating systems support FireWire.
It is common for a computer to have several USB 2.0 connections and at least 1 FireWire connection located very near the USB ports. This can help if a user needs to locate their FireWire port.
This is what a 6 pin and a 4 pin FireWire connector look like: