The key components of Cisco ACI (Application Centric Infrastructure) include the Application Policy Infrastructure Controller (APIC), which serves as the central management and policy enforcement point for the entire ACI fabric. The fabric consists of leaf and spine switches that provide network connectivity, while the leaf switches also offer connectivity to servers and other end devices. ACI also incorporates the concept of overlay networking through the use of VXLAN tunnels, which enable the transmission of virtualized network traffic over the physical infrastructure. Additionally, ACI utilizes endpoint groups (EPGs) to categorize and group endpoints based on their roles or functions, enabling policy-based enforcement. The ACI fabric is designed to be programmable and enable automation through the use of open APIs, allowing for easier integration with other systems and tools. Overall, these key components work together to provide a unified and policy-driven approach to networking in modern data centers.