Software-Defined Networking (SDN) is a networking architecture and approach that separates the control plane (which makes decisions about where traffic should be sent) from the data plane (which actually forwards the traffic). SDN introduces a centralized controller that manages the network and makes decisions about how to route traffic. This separation of control and data planes provides several advantages, including increased network agility, flexibility, and programmability. Here are key aspects of SDN:

1. **Centralized Control:** In traditional networking, control functions are distributed across individual network devices. In SDN, the control plane is centralized in a software controller that manages the entire network.

2. **Programmability:** SDN allows network administrators to program and automate network behavior through software. This enables rapid configuration changes, network optimization, and the ability to adapt to changing requirements.

3. **Open Standards:** SDN is based on open standards and APIs (Application Programming Interfaces), making it vendor-agnostic and allowing for interoperability between different hardware and software components.

4. **SDN Controller:** The SDN controller is a critical component that acts as the "brain" of the network. It communicates with network devices (switches, routers, etc.) through southbound APIs and with applications through northbound APIs.

5. **Southbound APIs:** These interfaces allow the SDN controller to communicate with network devices, instructing them on how to handle traffic. Common southbound APIs include OpenFlow, NETCONF, and gRPC.

6. **Northbound APIs:** These interfaces enable communication between SDN applications and the controller. Developers can create applications that interact with the controller to control network behavior.

7. **Network Virtualization:** SDN can provide network virtualization, allowing for the creation of logical network overlays on top of physical network infrastructure. This is useful for multi-tenancy and isolation.

8. **Traffic Engineering:** SDN allows for fine-grained traffic engineering and control, optimizing network performance and resource utilization.

9. **Dynamic Policy Management:** Network policies can be defined and enforced at a granular level, allowing for dynamic security and quality of service (QoS) policies.

10. **Cloud Integration:** SDN is commonly used in cloud environments to provide dynamic network provisioning and automation to support cloud services and applications.

11. **Network Function Virtualization (NFV):** SDN can be used in conjunction with NFV to virtualize network functions such as firewalls, load balancers, and intrusion detection systems, making them more flexible and scalable.

12. **Slicing:** SDN enables network slicing, which is essential for 5G networks. It allows the creation of isolated, virtual network segments tailored for specific applications or user groups.

SDN is widely used in data centers, campus networks, wide area networks (WANs), and cloud environments. It provides greater network flexibility, scalability, and the ability to respond to changing network requirements rapidly. By separating control and data planes, SDN offers a more agile and programmable networking infrastructure. The Open Networking Foundation (ONF) and the OpenDaylight Project are among the organizations and initiatives contributing to the development and standardization of SDN.