Putting your Mac into Target Disk Mode basically just shuts down the rest of your computer, except the Storage Device (often a hard disk drive) and the FireWire ports, along with a few other necessary peripherals.
Which ever computer is put into Target Disk Mode will show up on the other computer as a simple external drive.
Any Mac with FireWire and Mac OS 8.6 or later can be put into Target Disk Mode.
To put your Mac into Target Disk Mode:
1) Shut Down the targeted Mac... Plug in the FireWire cable... turn on the Target Mac while holding the T key, until the FireWire symbol appears on the screen.
- OR -
2) Go to System Preferences (Located on your Dock, or Apple Menu > System Preferences).... Click on the Startup Disk icon on the 4th row... and click 'Target Disk Mode.'
To exit out of Target Disk Mode, make sure all data transfers have stopped. Eject the Targeted Drive by dragging the icon to the trash (it'll turn into an Eject symbol). Then press and hold the power button of the Target Mac. When you start it back up, it'll boot normally.
Hope this helps.