What is the Shared Reponsibility Model
An agreement between AWS and the user on what aspects of the architecture belong to which party. Dependent on the technology being used.
What is the Physical Level?
The iron and concrete where the servers run. AWS always manages this.
What is the Network Level?
The physical devices and configuration defining the AWS network. AWS always manages this.
What is the Hypervisor Level?
The hypervisor application hosting virtual machines used by the user. AWS always manages this.
What is the Guest OS level?
The actual operating system. For EC2, the user must manage this.