In a nutshell, SDN is a way of separating the control plane from data plane physically. Whereas a router house both, when you start doing sdn, its like spliting the router into two. Control plane in networking is the the heart of everything where protocols stay. The data plane is where data gets actually moved once a protocol which resides in the control dcides which is the best path the data can be forwaded.
SDN is not really new, if you are running Wireless networks, there is a form of sdn network there. Two parts are found in wireless network, the controller and Access Points. This is a separation of duty which is what makes sdn.
Every vendor have a name for it like Cisco calling it ACI, Juniter calling it Contrail and some calling a form of NFV.