Web 1.0 was about finding information. Web 2.0 was about giving the everyday user ways to create interactive web experiences - websites, calendars, picture galleries and other collaborative tools. "Rather than a search engine, it [Web 3.0] is an 'answer engine' that interprets actual questions and answers them in accordance with their intended meaning. The Futurist compares its significance to the launch of Netscape in 1994. It's dubbed "the Semantic Web" because it's all about asking questions in ways that will give you useful answers. (WolframAlpha : The Birth of Web 3.0, The Futurist, May 7, 2010)
So, what is an "answer engine"? Steven Wolfram demonstrates how such an engine could calculate, for a particular city in the US, what the weather was on a particular day, compared to the weather in London, England on that same day. You ask the question in plain Enlgish and Wolfman Alpha uses databases, algorithms and other existing ways to crunch numbers to give you an answers that makes sense.