ping command

[tbaher]

It is a way to measure latency (lag) between you and whatever you choose to ping.
Let's say you ping www.yahoo.com by typing

ping www.yahoo.com

it will say something like

Reply from 216.109.118.77: bytes=32 time=72ms TTL=52

This means the amount of time a piece of data (32k by default) is sent from you to it and back is 72 milliseconds.
it is useful in 2 ways .. to see if you can reach a certain host (otherwise it would say "request timed out" if u couldnt) and how far away that host is.. latency-wise. Like when playing a game on a server for instance, you know how much you will be delayed therefore how playable the game is