Kubelet

The kubelet is the agent that runs on each node, and is responsible for launching pods. It doesn't directly run containers but instead controls a runtime, such as Docker or rkt. Typically, the kubelet watches the API server to discover which pods have been scheduled on its node.

The kubelet operates at the level of PodSpec, so it only knows how to launch pods. Any of the higher-level concepts in the Kubernetes API are implemented by controllers that ultimately create or destroy pods with a specific configuration.

The kubelet also runs a tool called cadvisior that collects metrics about resource usage on the node, and using each container that is running on the node, this information can then be used by Kubernetes when making scheduling decisions.