All a ping is a return of your traffic. You send a small amount of traffic out and the return is the site saying here I am and I am up. Just like ping pong, you send the virtual ball out and the site sends it back to your pc. What it does is send out the small piece of data from your machine out through the many routers and firewalls from here to there until it hits the destination, then upon complete traverse of the route it comes back to you as up or down. Try running a trace route for more detail of the route it is taking. Command in Windows is - tracert "destination" .
Hope this helps.