The best pizza analogy I could come up with is:

In a traditional web server, the person making pizzas only makes one at a time and can't do anything else until that pizza is done and delivered. In an evented model, each pizza maker makes many pizzas at once, and just watches for events (the oven timer goes off) and acts on them. (takes the pizza out) Sometimes if a task is taking him too long, he'll ask an assistant to work on it so that he can get back to making pies.