Ping (also known as your latency) is a measurement of the time (in milliseconds) it takes for a packet of information (a mouse click 4 example) to transfer from your computer to the game server (or vice versa).
So, from what I understand, it is basically the time it takes for your mouse click (saying I shot/knifed ect.) to reach the server you are playing on and to "register" on your target. I'm especially interested in this because at the current time I am running off a dial-up (56K) internet connection. I hope to get a faster internet soon, but for now its what I got...
I just wanted to post this because I think I have made like 5 knife kills in all of my COD4 playing time. I firmly believe that it is ping (along with some skill) that make you good (or crap) at COD4. Some people I come across don't think this is the case, but I (after dong some research and having experience with bad pings) think this is what can make or break you at COD4.
You can call me lame for actually making a post about this if you want, but I think it is an interesting topic and one of the things that gets swept under the rug when it comes to COD4.