MagicPacket is a format specified by AMD for waking up remote computers via WakeOnLan.
It is fairly simple - it can be included in the payload of any type of packet, as long as it will be seen by the machine (ie, a unicast packet to a third machine won't help if you are on a switched ethernet).
The format consists of 6 repititions of 0xFF, followed by 16 repititions of the IEEE address of the machine you are trying to wake up. On an ethernet, this refers to the MAC address.
See AMD's notes on MagicPacket 
here
etherwake is a tool to send MagicPackets.
See WakeOnLan, WakeOnLanNotes
One page links to MagicPacket:
