You could google what you need to know. The web is your resource library.
In the gist, Web 2.0 is a trend where content are user-driven. They decide what they want to see/hear. Web2.0 sites become a medium, channel that gathers groups of people together, and the direction and flow of the site very much depends on the users themselves. Of course, the sites will provide the relevant tools and apps.
Facebook provides the apps, the users can now develop their own apps. Decide what they content they wanna see.
Wikipedia provides the medium, users step in and fill the content. Same goes for StumbleUpon, Digg, LinkedIn and many many others.
And yeap, advertising brings good regenerative income on forums when you have a significant group of users, you can bring in the advertisers for targeted ads.
Forums could also be proactive and source for tie-up with content providers (relevant to the topics in the forum) and earn from the marketing/ads. Providers such as training, seminars (For entrepreneurs), fashion, makan places etc.
