Web 2.0 is a concept that takes the network as a platform for information sharing, interoperability, user-centered design and collaboration on the World Wide Web.
Web 2.0 is a loosely defined intersection of web application features that facilitate participatory information sharing, interoperability, user centered design, and collaboration on the World Wide Web.
Web 2.0 is the popular term for advanced Internet technology and applications including blogs, wikis, RSS and social bookmarking. The two major components of Web 2.0 are the technological advances enabled by Ajax and other new applications such as RSS and Eclipse and the user empowerment that they support. Web 2.0 is meant to take the Internet we know and turn it into a more interactive, more fun and friendlier Internet. Also, Web 2.0 is a title which is associated with web applications which facilitate user of the web application with the given attributes.
In a very simple word, web 2.0 is a category of web page where user or reader of that page can contribute anything like make comments, give thanks, etc. In that sense every blog that allows you to make comments are known as web 2.0 and of course every forum is a web 2.0