What is SDN?
Software Defined Networking (as defined by Wikipedia) is an approach to build computer networks that separates and abstracts elements of these systems. The origins of SDN come from the fact that the network has become increasingly complex to manage & operate. The original conclusion was that as a result of network devices being closed and innovation was limited to what a vendor offered. So if the control plane (configuration & control portion) could be separated from the data plane (packet forwarding portion) then they could presumably evolve independently, and be open to innovation from anyone.
Martin Casado Standford passout is considered as one of the fathers of SDN.

1.What is OpenFlow?
OpenFlow is a result of the PhD thesis of Martin Casado as advised by Nick McKeown & Scott Shanker. It is a communications protocol that enables a remote (i.e. Controller) entity to program the forwarding entries in a router or switch. Simply put, OpenFlow allows a network administrator to program forwarding policy of the network through a Controller. OpenFlow is controlled by the ONF (Open Networking Foundation) which is a non-profit organization.