what is ping?
Think of the web server as a submarine, sending out a pulse of sound (or in this case data). This is recieved by your computer, and it tells your computer to send it back. So it does. Meanwhile, the web server is timing the whole event, from the time it sends the pulse, to the time it recieves it. The difference is the 'ping'
Ping is actually derived from sonar, referring to the time it takes for the sound from the sonar to reflect off the enemy's hull and return.
For a more techy explanation, see here. (http://en.wikipedia.org/wiki/Ping)
(http://www.speedtest.net)