OpenFlow is a specification now managed by the Open Networking Foundation, which defines the functions and protocols used to centrally manage switches via a centralized controller. OpenFlow is a command and control protocol that includes communication over SSL/TLS protected channels, feature discovery and configuration of devices by the controller, and managing the forwarding tables on the switches. The OpenFlow protocol doesn't stipulate how the network is designed or managed. That is up to implementers and vendors to decide.
OpenFlow was also designed to work with existing products--no specialized hardware is required. A number of vendors are offering experimental hardware that runs OpenFlow today and can run both OpenFlow and their native switching/routing software on the same switch by dedicating specific ports to OpenFlow and native switching/routing. Two vendors, Fujitsu and NEC, are shipping OpenFlow switches. Fujitsu's switch is OpenFlow-only, while NEC's is a hybrid.
Greg Ferro produced a Practical Introduction to Applied OpenFlow video that is worth watching and goes a bit more in-depth into the OpenFlow protocol.