The accepted answer is correct, however here are the steps without having to leave this forum.
Connect the two computers with a FireWire or Thunderbolt cable.
On the Mac you want to use as the disk in target disk mode, do one of the following:
If the computer is off, start it up while pressing and holding the T key.
If the computer is on, choose Apple menu > System Preferences, click Startup Disk, then click Target Disk Mode.
When the computer has started up, a disk icon appears on the desktop of the other computer.
Transfer files by dragging them to and from the disk.
Eject the disk by dragging its icon to the Trash.
While you drag, the Trash icon changes to an Eject icon.
On the Mac you used as a disk, push the power button to shut it down, then disconnect the cable.