A shared network can be accessed by virtual machines that belong to many different accounts. Network Isolation on shared networks is accomplished using techniques such as security groups (supported only in basic zones in CloudStack 3.0.3).
Shared Networks are created by the administrator
Shared Networks can be designated to a certain domain
Shared Network resources such as VLAN and physical network that it maps to are designated by the administrator
Shared Networks are isolated by security groups
Public Network is a shared network that is not shown to the end users