The technology infrastructure of Web 2.0 is complex and evolving, but includes server software, content syndication, messaging protocols, standards-based browsers, and various client applications. (Non-standard browser plugins and enhancements are generally eschewed.) These differing but complementary approaches provide Web 2.0 with information storage, creation, and dissemination capabilities that go beyond what was formerly expected of websites.
A website could be said to be built using Web 2.0 technologies if it featured a number of the following techniques:

Technical:
CSS, semantically valid XHTML markup, and Microformats
Unobtrusive Rich Application techniques (such as Ajax)
Java Web Start
Flex/Laszlo/Flash
XUL
Syndication of data in RSS/Atom
Aggregation of RSS/Atom data
Clean and meaningful URLs
Support posting to a weblog
REST or XML Webservice APIs
Some social networking aspects

General:
The site should not act as a “walled garden” – it should be easy to get data in and out of the system.
Users should own their own data on the site
Purely Web based – most successful Web 2.0 sites can be used almost entirely through the browser
Applicable to an emerging generation of game development, proposed as Thin games

The term was coined by Dale Dougherty of O’Reilly Media brainstorming with Craig Cline of MediaLive to develop ideas for a conference that they could jointly host. Dougherty suggested that the Web was in a renaissance, with changing rules and evolving business models. Dougherty gave examples — “DoubleClick was Web 1.0; Google AdSense is Web 2.0. Ofoto is Web 1.0; Flickr is Web 2.0.” — rather than definitions. He recruited John Battelle for a business perspective, and O’Reilly Media, Battelle, and MediaLive launched the first Web 2.0 Conference in October 2004. The second annual conference was held in October 2005.

In their conference opening talk, O’Reilly and Battelle summarized key principles they believe characterize Web 2.0 applications: The Web as platform; data as the “Intel Inside”; network effects driven by an “architecture of participation”; innovation in assembly of systems and sites composed by pulling together features from distributed, independent developers; lightweight business models enabled by content and service syndication; the end of the software adoption cycle (“the perpetual beta”); software above the level of a single device: leveraging the power of “the Long Tail.”

So the Web 2.0 Conference hosted by O’Reilly is just finishing up today. Om Malik posted some updates from the conference here and here on his weblog. If you’re not sure what this thing was all about, the continue reading.

Only the Web 2.0 Conference brings the intelligence, innovation, and leadership of the internet industry together in one place at one time. You’ll learn what business models are working, what’s next on the horizon, and how all of this will affect your own business. We’ve built in plenty of time for catching up with old friends and making new acquaintances, and for connecting with the leaders and technologists redefining the Web’s business opportunities.

Once again we’ll be limiting attendance to maintain an intimate setting and foster dialogue among all participants. Request an invitation now, and come be a part of the conversation.

Web 2.0 is brought to you in partnership with O’Reilly Media, Inc. and MediaLive International and moderated by John Battelle, Program Chair, and O’Reilly CEO and founder, Tim O’Reilly.

Web 2.0 refers to the transition of the World Wide Web from a collection of websites to a computing platform serving web applications to end users. Ultimately, Web 2.0 services will replace desktop computing applications for many purposes. I think of it as the ultimate thin client application