Can you explain the differences between original domain, mapped domain, domain entered by user?

Can you explain the differences between original domain, mapped domain, domain entered by user? See: http://screencast.com/t/qDSSFhQET

It affects the wpmu dev videos. I want my customers to be able to create their own subdomain name. If they don't want to use the sub-domain, then I want them to be able to use their own domain. How does that affect the WPMU dev video plugin?

I once asked something similar but not as detailed. Haven't found that previous question.