Software-defined networking (SDN) is becoming a huge deal. To many people, the term is opaque, almost to the point of being meaningless. After all, what part of anyone's network isn't software-dependent? Every firewall, router, and switch you run has software (firmware) to control it. But with SDN, the management and control planes aren't the only ones implemented in software -- the bulk of the data plane is as well.
Among the variety of important ramifications, one in particular stands out: With SDN, you're using commodity server hardware (typically on top of or within a virtualization hypervisor) to manage, control, and move your network's data. This is different from the pre-SDN approach of running management and control software on top of purpose-specific ASICs (specialty chips) that move the bits to and fro. This means you can deploy entire new network components, configure them, and bring them into production without touching a screwdriver or a piece of sheetmetal, thanks to SDN.
Early days for SDN
SDN is obviously popular in the context of server virtualization. The first SDN in fact might have been EMC VMware's vSwitch -- a simple way of isolating Layer 2 network segments in a virtualization host. Since then, SDN has grown to include virtualized firewalls, routers, fully functional switches, and intrusion detection and prevention systems (IDS/IPSs) -- essentially anything you deploy on your physical network, but run virtually.