Below is an interesting video from a conference showing the value of a SDN network. The author shows what mean SDN as well as network virtualization. It does it by comparing it to the server virtualization as we know today and gives examples base on the Arista product lines.
What differentiate this video from others is the author's humerus way of presenting it. You get once in a while a funny comment or an interesting slide so you don't fell asleep :) Arista Networks SDN Essentials Seminar.