Jonathan Frieden (who runs the e-commerce law blog) has a nice, pithy summary of Section 230:
If the "essential published content" is willingly provided by a third-party, the interactive computer service provider publishing that content enjoys the full immunity afforded by Section 230.
Section 230 is the very cornerstone of Internet Freedom, the law that makes possible Robert Nozick's "framework for utopias": Online communities ("utopias") can flourish in their infinite variety only because those who build, host or enable access to such communities (social network operators, search engines, aggregators, etc.) do not have to worry about legal liability for user-generated content. The fundamental difference between Web 1.0 and Web 2.0 lies in the movement of online speech away from individual websites where the speaker was operator to online speech platforms where the potential number of speakers is essentially unlimited. This ongoing shift makes Section 230 more important than ever.
Never before has it been so easy for users to "vote with their feet," sorting themselves into communities of their own choosing, and not since the the 1890 Census declared the American frontier "closed" has it been been so easy for the individual to start entirely new communities if they don't like their current options.