FireWire was a proprietary Apple technology which allowed for incredibly fast transfer speeds between devices. Indeed, it was one of the features that made the original iPod so compelling. Beyond that, FireWire was, for a time, the de-facto standard for transferring digital movie footage to Macs.
Unfortunately, Apple ultimately began phasing out FireWire on Macs in 2008 as transitioning to USB expanded the company's pool of potential users. It's a shame, though, because USB 2.0, while decent, was vastly inferior to FireWire. The staggered abandonment of FireWire ultimately gave way to Thunderbolt.