Ping ("Packet InterNet Groper") is the simplest type of activity you can have on the Internet. With ping, you send out a single "echo" packet to the target, which then responds with a simple reply. You can then time the response to see roughly how far away the system is: the more "hops" (routers) and the slower the lines, the longer it takes for the ping to return. In fast-paced games, such as Doom and Quake, ping is important because it measures "lag": the time between when you send a command to the server, it processes it, then sends the updates back to your computer. If the lag is too long, then the game becomes unplayable: the screen doesn't move until long after you pressed the keys.
Ping is also used by hackers/crackers to scan the Internet looking for machines that are "alive". Most IP addresses are used, so hackers use pings to "map" the Internet looking for machines that are alive. Once such machines are found, the hacker may then carry out further attacks on those machines.
Ping is based upon the ICMP protocol, which runs "under" the transports TCP and UDP. This means that a firewall protecting ports on a system may still respond to a ping.
