Here I found a piece of description which serves the best about what Web 2.0 actually means:

In addition to favoring simpler solutions built by leaner teams, the stuff labeled "Web 2.0" tends to have technological commonalities.

On the back end, it is most often powered by open source technologies like PHP or (especially) Ruby on Rails.

On the front end, it is mainly built with web standards—CSS for layout, XML for data, XHTML for markup, JavaScript and the DOM for behavior—with a little Microsoft stuff thrown in.

When web standards with a little Microsoft stuff thrown in are used to create pages that can interact with the server without refreshing, the result is web apps that feel peppy and, dare we say it, Flash-like. In a white paper that actually got read, writer/consultant Jesse James Garrett named what I've just described. He called it AJAX, and the acronym not only took, it helped interactivity powered by these technologies gain traction in the marketplace.