When a packet arrives at a switch in a conventional network, rules built into the switch's proprietary firmware tell the switch where to forward the packet. The switch sends every packet going to the same destination along the same path -- and treats all the packets the exact same way. In the enterprise, smart switches designed with application-specific integrated circuits (ASICs) are sophisticated enough to recognize different types of packets and treat them differently, but such switches can be quite expensive.
In a software-defined network, a network administrator can shape traffic from a centralized control console without having to touch individual switches. The administrator can change any network switch's rules when necessary -- prioritizing, de-prioritizing or even blocking specific types of packets with a very granular level of control. This is especially helpful in a cloud computing multi-tenant architecture because it allows the administrator to manage traffic loads in a flexible and more efficient manner. Essentially, this allows the administrator to use less expensive, commodity switches and have more control over network traffic flow than ever before.
SDN is sometimes referred to as the "Cisco killer" because it allows network engineers to support a switching fabric across multi-vendor hardware and application-specific integrated circuits. Currently, the most popular specification for creating a software-defined network is an open standard called OpenFlow. OpenFlow lets network administrators remotely control routing tables.