FireWire 800 is Apple's response to USB 2.0/3.0. It is backward compatible with FireWire 400 but the increased physical size requires an adapter (this is an important difference from USB as their connectors are also physically backward compatible). When FireWire 400 came out it promised huge transfer speeds compared to USB 1.0 which is why many camcorders even today use FireWire connections. Since USB 2.0 came out, it has dominated the marketplace (excepting the aforementioned cameras) so Apple decided to develop a sort of "FireWire 2.0" of their own. Thus was born FireWire 800. It has been largely ignored in the marketplace which is why it was surprising when Apple returned it to their product line. Again, some camera manufacturers have decided to implement it but support is limited. The new speed boost required new pins to be added and a new connector was developed.

If you need any more info, I suggest reading Wikipedia's article on it: http://en.wikipedia.org/wiki/FireWire#FireWire_800_.28IEEE_1394b-2002.29

Firewires are mostly used for streaming data. Because it is much faster than USB, it is prefered when transfering files.

The FireWire 800 (IEEE 1394b), the next generation of FireWire technology, doubles the throughput of the original FireWire interface to 800 Mbit/sec and dramatically increases the maximum distance of FireWire connections!

There is more about it on this site:

http://www.firewire-1394.com/what-is-firewire-800.htm