Kubernetes is an open source platform for running containers in production, providing more functionality than the Docker daemon alone. It distributes Docker containers over multiple hosts, using lessons learned from Google's infrastructure.
A container is a Docker container (because Kubernetes doesn't work with any other containers right now).
A node is a virtual machine (or a physical machine).
A pod consists of containers colocated on the same node.
A cluster is a collection of nodes, usually running in the same datacenter.
Slightly Less Basic Terms
A service is a load balancer.
A replication controller determines how many instances of a pod should be running at any given time.
A volume provides storage that lasts beyond the lifetime of a container.
A secret provides a volume to hold sensitive information (passwords, OAuth tokens, SSH keys).
A namespace provides a layer of isolation between Kubernetes objects.
Kubernetes objects are units of configuration that can have multiple types - every term introduced above is a Kubernetes object, except for containers and clusters.