the second generation of the World Wide Web in which content is user-generated and dynamic, and software is offered that mimics desktop programs
Web 2.0 encourages collaboration and communication between users.
A loosely defined term for web applications that go beyond displaying individual pages of static content and allow a community of users to interact with the site and each other by adding or updating the content.
Examples include social-networking sites like Facebook and other web-based communities, hosted services like Google Docs, web applications like GMail, video-sharing sites (Youtube), wikis (Wikipedia), web logs, mashups and folksonomies.
While Web 2.0 applications often use advanced web features like AJAX to improve the speed of interaction, the term is more about the type of applications than the technology used.
The term was coined by Darcy DiNucci in 1999, though she was discussing designing web sites for new hardware platforms.