[Skunkworks] What exactly is a "Private Cloud"?

Guys, I think am getting confused by the cloud jargons. What exactly is the difference between private cloud and datacenter? I am using the term data centre very loosely here to mean the corporate internal IT infrastructure There is a lot of talk about setting up private clouds yet in my opinion this has been there for ages. Doesn't corporate file server, exchange server, etc constitute a private cloud?