I'm blogging this because I've had to explain the differences a few times, and this does a better job than I could.
We've been having discussions at work about the promises and challenges of user-generated content. In that light, this article is quite timely:
"The estranged founder of Wikipedia, the online encyclopaedia written entirely by members of the public, is to launch a rival that he says is less likely to be riddled with errors.
Larry Sanger says that vast swaths of the anarchic encyclopaedia he helped create in 2001 are in desperate need of an editor – and that is what he is promising for his new project.
Mr Sanger has begun signing up academics furious at the mistakes and generalisations they find on Wikipedia's articles on their specialist subjects, and vowed to give these experts a special role to shape articles on Citizendium.org."