topics in OAuth about Protocol

rickwordy asked this question on the blog. I haven't seen XACML before, any one fancy a guess?

How does OAuth compare and contrast with OpenID?

I understand that OpenID provides a way for a user to provide a distributed authentication mechanism, which OAuth covers as well. I also believe that OAuth goes further in some respects by documenting things that OpenID leaves up to the providers. Is this accurate?

Are there any other ways that these two systems compare and contrast?