The specific phrase you're looking for is "Target Disk Mode". In Macs that came with firewire native, you hold down the "t" button after you hear the boot sound. Keep it mashed for about 5 to 8 seconds (for good measure) and then release. The machine in question should then display a big firewire logo that kinda bounces around the screen a bit.
At this point you now have an exceptionally expensive external firewire drive. Hook up stated Mac to the other Mac with a 6-pin to 6-pin firewire cable and the drive should mount up just like any other external firewire disk.
For added fun, you can actually boot your firewire native Mac off of any external firewire drive such as an iPod or that Mac you just put into Target Disk Mode. Try it, it's neat!
From that point, you can do all sorts of wacky things with Disk Utility or just start copying large wads of files. If you want to keep the external disk bootable then making a duplicated of the disk with Disk Utility or the dd command if you're Unix hardcore.