Assuming the data you wish to transfer is on your Apple laptop, turn off the laptop.
Connect a firewire cable (400 or 800 respectively) from the laptop to the desktop computer.
Power-up the laptop and depress the (T) key while it starts up.
Once the laptop has started, there are two indications that it has successfully been mounted on the desktop:
1) Your laptop will have a "bouncing firewire icon" on the screen
2) Your laptop will be mounted on the desktop as an external hard drive
Now that everything appears to be in working order you can begin to "drag & drop" files from your laptop to the desktop just as you would any other drive on the desktop computer.
It's that simple.