An Internal Developer Platform (IDP) is a platform layer that is built and shipped as a product by a dedicated platform engineering team to remove complexity without removing context, enabling developer self-service and true DevOps at an enterprise scale. It consists of five planes: Developer Control Plane, Integration and Delivery Plane, Monitoring and Logging Plane, Security Plane, and Resource Plane, with a Platform Orchestrator acting as the glue that makes them work together. Key design principles include giving developers their choice of interface, not breaking their workflows, and shipping the platform as a product. An IDP is distinct from an internal developer portal, which is just one part of the overall platform engineering space. A PaaS (Platform as a Service) can be a solution for small teams or greenfield scenarios but doesn't scale to complex enterprise setups and often lacks key features like code being the single source of truth and developers picking their interface of choice.