Excerpted from the manual of my latest toy, a new firewall appliance:
An easy way to visualize how security zones work is to imagine a large new building, with several rooms inside the building, and a group of new employees that do not know their way around the building. This building has one or more exits, which can be thought of as the WAN interfaces. The rooms within the building have one or more doors, which can be thought of as interfaces. These rooms can be thought of as zones inside each room are a number of people. The people are categorized and assigned to separate rooms within the building. People in each room going to another room or leaving the building, must talk to a doorperson on the way out of each room. This doorperson is the inter-zone/intra-zone security policy, and the doorperson’s job to consult a list and make sure that the person is allowed to go to the other room, or to leave the building. If the person is allowed (i.e. the security policy lets them), they can leave the room via the door (the interface).
I'm sure I'll get several "well duh, of course it's that easy. Any moron would see the analogy." But that's only because you're all a lot smarter than I am. Me? My eyes glazed over and I started visualizing going after all these people with a rocket launcher.