The technological infrastructure behind Web 2.0 is complex.
To quote directly from Wikipedia: http://en.wikipedia.org/wiki/Web_2.0#Technology_overview
"The complex and evolving technology infrastructure of Web 2.0 includes server-software, content-syndication, messaging-protocols, standards-based browsers with plugins and extensions, and various client-applications. These differing but complementary approaches provide Web 2.0 with information-storage, creation, and dissemination capabilities that go beyond what the public formerly expected of Web sites.
A Web 2.0 website typically features a number of the following techniques:
Ajax-based rich Internet application techniques
Non-Ajax-based rich Internet application techniques
CSS
Semantically valid XHTML markup and/or the use of Microformats
Syndication and aggregation of data in RSS/Atom
Clean and meaningful URLs
Extensive use of folksonomies (in the form of tags or tagclouds, for example)
Weblog publishing
Mashups
REST or XML Webservice APIs "