LATER: Boris pointed out that I forgot to link to the Wikipedia entry. Herewith the Wikipedia entry for Web 2.0.
I would define Web 2.0 (as I have defined Web 2.0 in the past) as a web site or web app (there's no difference in 2005) that that has the following characteristics:
- Clean URLs (ampersands, questions marks, etc. are not needed in the URLs of any "page" on any website)
- Standards based code (CSS, HTML, XHTML etc). This enables machine processing of the website as well as accessibility (although of course accessibility doesn't come automatically from using standards based code!)
- Syndication using RSS, Atom or any other standard syndication format
- Aggregation using RSS, Atom or any other open standard syndication format
- Clean Interfaces and APIs to get data in and out of the site
- Dynamic
- Easy to update without any technical knowledge of CSS, HTML, etc.
There's no need to describe specific technologies like web services or WebDAV. Those flow naturally out of #5, clean interfaces and APIs.
From Read/Write Web: It's Official: Wikipedia Entry For Web 2.0:
"Web 2.0 defines a newer incarnation of the World Wide Web typified by the transition from the typical website hosting HTML/XHTML pages, to a platform that provides a point of presence (sometimes known as a Web portal), from which any of the following interactions may occur:
1. Syndication of content using XML based formats such as RSS, RDF, Atom, and others
2. Aggregation of content published using XML based formats such as RSS, RDF, Atom, and others
3. Publishing of invocation endpoints for XML based Web services (these may be of the SOAP/WSDL/WS-* variety of RESTian XML-RPC)
4. Conventional publishing of HTML/XHTML documents
5. Exposure of WebDAV based resources and collections"