Target disk mode
Target disk mode enables a Macintosh's primary hard disk to be used on another computer, typically with a FireWire 400/800 connection. The hard disk of the computer in target disk mode will usable on the second computer as if it were in an external FireWire drive enclosure. When someone is logged into the second computer, the Desktop will display the icon for the hard disk of the computer in target disk mode.
On newer Macintosh models (after approximately 2003), the internal optical drive can also be mounted if a CD or DVD disc is inserted. The disc can be ejected normally, using the Eject or F12 key on the keyboard - or through other means through applications (such as iTunes or the Disk Utility application).
Target disk mode is explain in Apple knowledgebase article 58583.
