“Web 2.0” refers to what is perceived as a second generation of web development and web design. It is characterized as facilitating communication, information sharing, interoperability, User-centered design and collaboration on the World Wide Web. It has led to the development and evolution of web-based communities, hosted services, and web applications. Examples include social-networking sites, video-sharing sites, wikis, blogs, mashups and folksonomies. Read more in Wikipedia.
In the early days of the web, web sites consisted of static pages, which severely limited interaction with the user. In the early 1990’s, this limitation was removed when web servers were modified to allow communication with server-side custom scripts. No longer were applications just static brochure-ware, edited only by those who knew the arcane mysteries of HTML; with this single change, normal users could interact with the application for the first time.
This is a huge and fundamental step towards the web as we know it today. Without interactivity, there would be no e-commerce (such as Amazon), no web e-mail (Hotmail or Gmail), no Internet Banking, no blogs, no online share trading, and no web forums or communities. The static Internet would have been vastly different to today.
As I say web 2.0 first thing that comes in my mind is Facebook because I think it’s very interactive web application, also Yahoo, Gmail, Youtube , wordpress and so on . All this web sites (web applications) have a lot of client side code (AJAX), they are distributed applications , users can interact with web site and each other in different ways and so on.
In this video Andi Gutmans Zend co-founder, explains what is web 2.0.
Myspace Tutorial: What is Web 2.0
You might would like to know what will be web 3.0 
good question!
Eric Schmidt Chairman and CEO of Google Inc. answers what will be web 3.0
Filed under: AJAX, ASP.NET, Google, javascript, PHP, www Tagged: | web 2.0, what is web 2.0
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