Cluster = the whole Kubernetes installation. Has a control plane + worker nodes.
Node = a single machine (physical or VM) that runs containers. Worker nodes run your pods; control plane nodes run the API server, scheduler, etcd, controller manager.
Namespace = a logical partition inside a cluster. Used for separating environments, teams, or apps. Default namespaces: default, kube-system, kube-public.
Pod = the smallest deployable unit. A pod contains one or more containers that share network and storage. Most pods have one container. Sidecars are additional containers in the same pod (e.g., a logging agent next to the main app).
Container = a running instance of a container image (Docker, OCI). The pod is the wrapper around containers.