In a classical router or switch, the fast packet forwarding (data path) and the high level routing decisions (control path) occur on the same device. An OpenFlow Switch separates these two functions. The data path portion still resides on the switch, while high-level routing decisions are moved to a separate controller, typically a standard server. The OpenFlow Switch and Controller communicate via the OpenFlow protocol, which defines messages, such as packet-received, send-packet-out, modify-forwarding-table, and get-stats.

via Intro to OpenFlow.

OpenFlow allows you to easily deploy innovative routing and switching protocols in your network. It is used for applications such as virtual machine mobility, high-security networks and next generation ip based mobile networks.