Traffic 2.0

The phrase Web 2.0 was originally coined three or so years ago and, unless you have been living on a far off planet or in a cave, no doubt you will have heard it.
Perhaps you might have wondered what it is all about, or indeed, whether Web 2.0 actually exists at all?
It may seem a little strange to be asking whether something that people have been talking about for three years exists or not, but the reason is that there is no easy-to-pin-down definition of what Web 2.0 actually is.