In conventional networks, routers, switches, firewalls and other devices tightly couple the control plane and the data plane on a single device. Software Defined Networking (SDN) introduces the concept of separating the control plane and data plane. The data plane remains on each device (physical and virtual) for quick, efficient forwarding of data. The control plane provides layer-2 MAC reachability and layer-3 routing information to network devices so they can make packet forwarding decisions.
The data planes' job is to forward user-generated data traffic within the network infrastructure. How that happens is device dependent.
In SDN, the data plane and control plane are separated. The functionality of a devices' data plane is dependent on instructions coming from the centralized controller's control plane.