I am taking an introductory to the cloud and I am writing a short paper on openstack. I have started looking into Openstack and understand it a bit, but I have some questions I can't find the answer to.

From what I understand, Openstack is an operating system that allows you to deploy a cloud environment on your in-house infrastructure. It is also used by rackspace. My main question is, since openstack is open source does it allow you to easily migrate your virtual machines from your private cloud to a public cloud? I think it would be easy to do this with rackspace, but what about AWS and the others?

Also I'm having trouble understanding what makes openstack more than just an OS that allows you to deploy a private cloud.