Traditional Networking vs Software Defined Networking

Software Defined Networking is a new paradigm of network control,  where the control plane is outside of the network device, that permits a global vision of the network and control it efficiently.

In the traditional paradigm of networking, the control plane and the data plane are in the same hardware  device, that make the device only "see" packets entering and packets forwarding, that, makes the data is controlled using the information received from the other, and difficults the configuration on live if the net becomes to be saturated, or needs new network requirements. When we need change a configuration, we must go  to the switches one by one and change the configuration.

But with the Software Defined Networking paradigm, the control plane is outside of the network devices (generally a server), allowing us the  centralized control of the network and letting us a global vision of the network. When the control plane needs to configure a device used for example open flow to send the new configuration required to every  switch.

In short SDN creates and dynamic and flexible network architecture that can change as the business requirements change.