SDN stands for Sofware Defined Networking, which is a way of controlling the behavior of data as it moves from you (the user) to each of the multitude of devices between you and wherever your data is going or wherever you are fetching it from.
The largest player in the SDN space is Cisco Systems, who build and control the majority of devices (and the software built-in to them) that are used on the Internet and within millions of devices and data centers globally that move your data.
And if you want to learn more about "the control plane" and "the data plane" I suggest you start with Wikipedia and move on from there.
The bottom line is that Cloud Computing and "virtualization" have decoupled the hardware devices from their functions and permitted software controls to "impersonate" traditional switching and routing functions. These new technologies are some of the reasons that Cloud Computing is so powerful, scalable, and inexpensive.
Cisco and the rise of SDN
So we'll continue to investigate some of the technical underpinnings of the Cloud without, hopefully, glazing over your eyes in the process.
Thanks to The Cloud is Huge