Basically, it's an ability to dynamically define the control plane of your network. To a certain extent, this is also re-writing aspects of network protocols at a whim, perhaps based on the contents of a packet.
In a sense, it's a buzzword, because it is the natural progression of periodically running scripts that dynamically test your network and shape iptables rules accordingly.
SDN is a pretty average name for what it is. It's being able to write the features of your equipment and thus network, yourself, tailored for you. No dependency on vendors aside from complying with OpenFlow standards, for example.
- Ryu, Python based - Opendaylight Java based, vendor driven - Cumulus Linux, debian based - Pica 8 OS, debian based - Opennetlinux.org, debian based - Opencontrail, Juniper, Ubuntu based, network standards - Onos carrier grade open networking - Big Switch Switchlight based on opennetlinux, debian
List of SDN switches: - Edgecore - Quanta networks - Facebook 6pack - Arista networks(Fedora based, market leader)
Page with SDN switches - http://whiteboxswitch.com/collections/all-switches