IEEE 1394 is a high speed I/O made by Apple in 1995. Back then, USB had terribly slow data capacities, and Apple (being the video professionals that they are...or were) decided to manufacture a faster I/O so that things like video could be transferred faster.
You might have heard IEEE 1394 called Firewire (still used today) and/or i.LINK (Sony used this name, because they didn't want to be sued by Apple).
Facts about IEEE 1394
Firewire is mainly used for camcorders. You can fast forward, rewind, play, and pause through the Firewire interfase, but you can't do it though USB (interesting fact). They are commonly on external hard drives as well, and Apple uses FireWire for Target Disk Mode which lets you use an other Mac as a spare hard drive for all intensive purposes.
It comes in three different varieties: 4 pin, 6 pin, and 8 pin.
Data speeds range from 50-800 MB/s (4 pin = 50 MB/s, 6 pin = 400 MB/s, 8 pin = 800 MB/s)
You can daisy chain up to 63 devices
More information about IEEE 1394 can be found here: http://en.wikipedia.org/wiki/IEEE_1394
Aug 27 '11 at 19:05