A ping of death (a.k.a. "POD") is a type of attack on a computer that involves sending a malformed or otherwise malicious ping to a computer. A ping is normally 64 bytes in size; many computer systems cannot handle a ping larger than the maximum IP packet size which is 65,535 bytes. Sending a ping of this size often crashes the target computer.
This bug has traditionally been relatively easy to exploit. Generally, sending a ping packet of a size such as 65,536 bytes is illegal according to networking protocol, but a packet of such a size can be sent if it is fragmented; when the target computer reassembles the packet, a buffer overflow can occur, which often causes a system crash.
This exploit has affected a wide variety of systems, including Unix, Linux, Macs, Windows, printers, and routers. However most systems since the late 90's or so have been fixed, so this attack is mostly historical in nature.