Context: System Center Virtual Machine Manager 2012 server and Hyper-V 2012 host

A couple of quick questions:

1. Are the following equivalent: guest agent, host agent and VMM agent?
2. What does the VMM agent do? It is required for communication b/w the VMM server and the host? I know that this is a basic question but believe it or not, I haven't been able to find an explanation online...

Thanks

-Rohan.