It is totally different thing to design print and tv ads, simple web sites and banners, then it is to understand the complex world of interactive and consumer collaborative Web 2.0.
There is no single, clear, definition to Web 2.0, since it is much more then advancement of web technologies. John Seely Brown made good quote material when he said "Web 2.0 is a profoundly participatory medium". Tim O'Reilly, the man who originally coined the term Web 2.0, made an excellent list while trying to clarify what the term Web 2.0 meant:
Web 2.0 means more collaborative work, information sharing, inviting people to participate, giving them more power to voice their opinions. Sites and applications are more interactive, more rich in media, and give better user experiences.
Loosely quoting O'Reilly: Trust your consumers. Let them play. Participate, don't publish. Listen, don't shout. Remember, your site should get better if there are more people using it. And finally: Web 2.0 is an attitude, not just technology.