Target disk mode is a way of booting a Macintosh computer so that it acts as an external hard drive on another computer. When a Mac is booted in target disk mode, the typical boot sequence is bypassed and the operating system is not loaded. Instead, the computer's internal and external hard drives are simply mounted on a connected computer. Target disk mode can be used to manually transfer files between two machines or to copy data from one computer to another using Apple's Migration Assistant.
In order for target disk mode (TDM) to work, two Macintosh computers must be attached to each other via a Firewire cable. One computer should be on and the computer designated for TDM should be off. To boot into target disk mode, hold the "T" key on the keyboard immediately after turning on or restarting the computer. After a few seconds, the screen should display the Firewire icon, which will move around the screen as long as the machine is in target disk mode. You should then see the hard drive(s) of the computer in TDM appear on the Desktop of the connected computer.
Booting a computer in target disk mode makes it easy to transfer files between two machines. Since the hard drives of the computer in TDM automatically mount on the other Mac's desktop, you can simply drag and drop files between them. Also, the computer in target disk mode is not seen as a boot disk, so you don't have to worry about file permissions. You can also run more comprehensive disk diagnostics and repairs. Just be sure not to remove or copy over any important system files on the TDM hard drive(s), since there are no safeguards to protect you from doing so.
Since the hard drives of the TDM machine are mounted on the connected computer, you should make sure to unmount or "eject" the hard drives before you turn off the computer. This can be done by selecting the hard drive on the desktop and choosing "Eject" from the File menu. Once the hard drives are ejected, you can safely turn off the TDM computer. When you turn on the computer again, it should boot normally (as long as you don't hold down the "T" key). Any files you copied to the computer's hard drives should appear in the directories you copied them to.