Kubernetes Container Runtime Concepts
External
Internal
Container Runtime
Each Kubernetes node runs a container runtime, usually Docker. However, support for other container runtimes is available, via Container Runtime Interface (CRI).
The Container Runtime Interface (CRI) is an abstraction layer that standardizes the way third-party container runtimes interface with Kubernetes.
Runtime Classes
Runtime classes is a feature that was introduced in Kubernetes 1.2. It allows for different classes of runtimes (gVisor, Kata Containers, etc).
Docker
Kubernetes and Docker are complementary technologies. Docker is the technology that starts and stops containers under Kubernetes' supervision. Kubernetes is the higher-level technology that decides which nodes to run containers on, when to scale an application up or down or when to apply updates.
containerd
A container runtime that consists in a stripped-down version of Docker.