We like using analogies here to explain security principles. For example, to explain why your firewall must truly protect every port and pass every leak test, we point to a car with only 3 of its 4 doors locked. How safe is that? That also works for pointing out why you need a complete security suite, not just antivirus.
Sky King, our fearless leader for ZoneAlarm development, likes to use a different analogy to point out the value of multiple layers of security. Let’s say you want to build a castle in medieval times. First, you build on top of a hill. That way, as attacking forces approach, you stop a lot them as they climb, perhaps with trained archers and elves (sorry, I sometimes get Lord of the Rings and “real” history confused). Next the forces of Saruman have to cross a moat (water, spikes, more arrows), scale a wall (hot oil from above) or break through a gate (heavily fortified), defeat the forces in the towers and top of the walls (hand to hand), and then enter the center of the castle. By then, you will have defeated all of the dark forces (mixing metaphors with Star Wars) and can live in peace. It’s probably a good idea to add intelligence services to catch spies who slip in un-noticed (James Bond movies are cool, too) or use your spider-sense (couldn’t resist).
The moral of the story? No single layer of defense is perfect or sufficient by itself. Some fun sites on medieval castles include http://en.wikipedia.org/wiki/Castle, http://www.castles-of-britain.com/castleso.htm, http://tolkiengateway.net/wiki/Battle_of_the_Hornburg.