"SDN is a new approach to networking in which network control is decoupled from the data forwarding function and is directly programmable. The result is an extremely dynamic, manageable, cost-effective, and adaptable architecture that gives administrators unprecedented programmability, automation, and control. Implementing SDN via an open standard enables extraordinary agility while reducing service development and operational costs, and frees network administrators to integrate best-of-breed technology as it is developed."
Source - Open Networking Foundation
What is behind these terms?