Software Defined Network decouples the control plane from the data plane. It also decouples the control plane from the underlying network hardware and implements it in software instead; this enables programmatic access to the network enabling greater flexibility in network administration. Moving the control plane into software provides dynamic access for administrative tasks. A network admin can shape the traffic from a central control console without having to touch individual switches. An admin can prioritize, deprioritize and even block certain kinds of data packets with very granular level of control by chaning the rules from a central command console.
An overlay network is a telecommunications network that runs independently on top of another one although supported by its infrastructure.
Data plane, control plane and management plane are the 3 different areas of operations and carry different kinds of traffic. A plane conceptually (and often in reality) is an overlay n