The thesis I completed for my Master's degree was on a Web 2.0 technology, and I frequently get asked by technical people and non-techies alike exactly what Web 2.0 is. Most folks know that Web 2.0 makes the old web look prettier and "slicker" and has something to do with community, but many are not sure of how to distinguish between the two.
Problem is that Web 2.0 isn't exactly anything. It's fuzzily many things. So here is my current analogy:
Web pre-2.0 : dinner at a friend's house :: Web 2.0 : a potluck dinner
I like this analogy because it addresses the inherent fuzziness in trying to define something that is an evolution of its previous existence. First, I'll explain the dinners, then try to relate them back to the different Web x.x's.
When you go to a friend's house for dinner, your friend makes dinner for you...most of the time. Maybe you brought over some brownies for dessert, and maybe he ordered in an appetizer from the local Indian place, but basically, he is presenting dinner.
When you go to a potluck dinner, everybody brings a dish (except those sneaky little lurkers who just show up and help themselves while contributing nothing). You probably would not have eaten 0l' Peggy's meatloaf had you just run into her on the street, but there is an inherent sense of trust at this location, so you help yourself to a large slice. You like Jerry's mini-quiches so ask him for the recipe, which he readily gives you (unlike Lenny who encouraged you to eat all of his rice pilaf you like but would not tell you how it was concocted). You talk to Ann about her potato salad and end up making plans to play tennis in a week.
The Web pre-2.0 is very much like a friend's for dinner. You are going to a specific place for a specific purpose. You used to go to your bank web site to check your account, just as you go to your friends to eat your dinner. Maybe as the web evolved, you began to pay bills there. This made it a two-way street: instead of just getting information, you were giving some back. This is kind of like you bringing a dessert to contribute to a dinner.
On to our potluck. Imagine a typical potluck dinner, maybe at a community center. You look around the room and see all kinds of different people. You wonder which food they bought. You see some people that look out of place or that you may not trust, but most people in the room have earned some kind of trust from you by sharing the communal purpose of a potluck. Some folks are there to see others, some are there to promote services (like the local injury lawyer), some are there to watch you.