Not long ago I had Google Adwords described to me like this:
Let’s pretend for a moment that you sell new TV’s.
Think of Google Adwords as a magical highway that only lets people on the road who need a TV.
Well, I can sell you the billboards all along that highway, but I won’t charge you for impressions (the traffic passing by), I’ll only charge you for clicks by people who want to see what you have to offer…
Pretty cool huh?