FireWire (also known as IEEE 1394 or iLink) is for connecting external devices like hard disk or audio/video interfaces or video cameras to a computer.
It has its own controller on the motherboard/logicboard, contrary to USB (which uses the CPU), and can sustain data rates from 35 MB/s (FW 400) to almost 70 MB/s (FW 800) constantly.
Every Mac since the end of the 90ies had one of those, except now the MacBook (Unibody).
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