For better or worse, the term "Web 2.0" has become the umbrella for describing a lot of concurrent trends that are coalescing to enable a richer online experience. Like all good umbrella concepts, the term has become overloaded to the point of being almost meaningless.
With an eye towards putting some meat on the Web 2.0 bones, I recently wrote a post on my blog for O'Reilly that is called, "What the hell is Web 2.0? The great web mash-up begins."
In the post, I attempt to define the atmosphere that Web 2.0 applications operate within, make (hopefully) clear assertions about what the moving parts are, and finally, the core attributes of those parts. The goal is to give the reader some "kindling wood" for thinking about the different types of composite application services that can be created around this emerging model. To the extent you are interested in such things, check it out.
Also, over at Read/Write Web, Richard MacManus has written an intriguing post called, "Fleshing out Web 2.0" that provides some worthwhile links to other Web 2.0 universe defining resources, including one called, "Map of the Web 2.0 World" that I thought was well articulated.
