There is a lot of talk about Web 2.0, my paper to CAADRIA even mentioned it in the title. Whether its a fad or the next big thing is fairly uncertain but it does provide a nice general purpose container for a bunch of different read/write Web concepts like blogging, tagging, and RSS. Certainly there is a lot of hype around the whole thing but as a general theme for a bunch of technologies it is pretty strong.
What I do think it is achieving is the evolution of a far more conceptually and technologically richer Web space. The Web is no longer about hugely expensive and flashy billboards, tightly controlled portals or online stores with weird names. Sure all these things still exist but they are now taking second place to far more dynamic sites with their background and identity rooted firmly in the Web rather than being simply the extension of a conventional organisation's operations. Web 2.0 seems also to be more about empowering the individual to be able to do things like get their own 'stuff' online and track/search the Web in ways that make sense to them. MySpace, Blogger.com, Technorati and Flickr are all successful Web 2.0 sites that have these fundamental ideas at their core. Recently Apple announced iWeb which is the first 'traditional' application that attempts to blur the lines between the computer, the Web and your life.
Personally I agree with Jonathan Boutelle, AJAX is not part of Web 2.0. If you do not know already AJAX (or to use the proper terminology Asynchronous JavaScript And XML) is a technique for updating a page without the user having to navigate to a different screen through a HTTP POST/GET request. The HTTP POST/GET requests still occur but in the background, usually tied to certain events on a page. AJAX provides a level of application richness that has been lacking from conventional HTML/Javascript but has always been a part of traditional programs. Consequently with its emergence we are seeing more Web applications that are mirroring the functionality of their traditional bretherin. Applications like GMail and Writely are beginning to replace Outlook and Word but this is merely a transition from an operating system based application silo to the more generic but still silo driven Web application space.
Tim O'Reilly has talked about the subject a lot in a couple of articles:
Plus his company helped organise a Web 2.0 Conference and many of the presentations can be listened to on IT Conversations.