Basically, the term encapsulates the idea of the proliferation of interconnectivity and social interactions on the Web.
Tim O'Reilly regards Web 2.0 as business O'Reilly considers that Eric Schmidt 's abridged slogan, don't fight the Internet, encompasses the essence of Web 2.0 — building applications and services around the unique features of the Internet , as opposed to building applications and expecting the Internet to suit as a platform (effectively "fighting the Internet").
The term Web 1.0 came into use after the evolution of the term Web 2.0. embracing the web as a platform and using its strengths (global audiences, for example).
“ … an idea in people's heads rather than a reality. It’s actually an idea that the reciprocity between the user and the provider is what's emphasised. In other words, genuine interactivity, if you like, simply because people can upload as well as download."
Stephen Fry , who writes a column about technology in the British Guardian newspaper describes web 2.0 as ....
According to O'Reilly and Battelle,an architecture of participation where users can contribute website content creates network effects . Web 2.0 technologies tend to foster innovation in the assembly of systems and sites composed by pulling together features from distributed, independent developers.
This could be seen as a kind of "open source" or possible "Agile" development process, consistent with an end to the traditional software adoption cycle, typified by the so-called " perpetual beta ".
LEVEL 3 LEVEL 2 LEVEL 1 LEVEL 0 can operate offline,but gain advantages from going online the most "Web 2.0" operate offline but gain features online work as well offline as online eBay, Craigslist, Wikipedia, del.icio.us, Skype, dodgeball, and AdSense Flickr Google Docs , Spreadsheets & iTunes MapQuest, Yahoo! Local,Google Maps
In November 2004 , CMP Media applied to the USPTO for a service mark on the use of the term "WEB 2.0" for live events.  On the basis of this application, CMP Media sent a cease-and-desistIrish non-profit organization [email_address] on May 24 , 2006 ,  but retracted it two days later.  The "WEB 2.0" service mark registration passed final PTO Examining Attorney review on May 10 , 2006 , but as of June 12 , 2006 the PTO had not published the mark for opposition. The European Union application (application number 004972212, which would confer unambiguous status in Ireland) remains currently pending after its filing on March 23 , 2006 . demand to the
>> The argument exists that "Web 2.0" does not represent a new version of the World Wide Web at all, but merely continues to use so-called "Web 1.0" technologies and concepts.
>>Techniques such as AJAX do not replace underlying protocols like HTTP , but add an additional layer of abstraction on top of them (more or less the same way as C++ language is an abstract layer on top of C language.
>>Many of the ideas of Web 2.0 had already been featured in implementations on networked systems well before the term "Web 2.0" emerged.
>> Venture capitalist Josh Kopelman noted that Web 2.0 had excited only 530,651 people (the number of subscribers at that time to TechCrunch , a Weblog covering Web 2.0 matters), too few users to make them an economically viable target for consumer applications.