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Web 2.0 is more like a milestone, it sets out a new generation of user interactivity. A lot of people think its a style, like "web 2.0" buttons, but it is actually sites like facebook/youtube/vimeo/ustream where the user of the site can interact in a way to upload their own content and create their own space and or upload their own content.
You would define the font size in the body tag in CSS to set a standard for the whole document. Because everything is inside the body the font size cascades unless specified at another point.
A small addition to Rob’s definition, it helps to think of Web 1.0 as an extension of the traditional publishing model of things like newspapers, books, radio and television… they are one-way forms of communication. The author creates the content then publishes/broadcasts it while the audience can only receive. Or put more simply, I talk, you listen.
Web 2.0 gives the audience the ability to not only participate in TWO-WAY communication (responding or replying to the content, often in real time) but often also to CHANGE or MODIFY the content itself. Wikipedia is the obvious example. A content creator publishes content, the public can interact with, change, or discuss the content on the fly. Very Cool.