A Docker Registry is a storage and distribution system for named Docker images. The Docker registry is used to store the static Docker image templates. The same image might have multiple different versions, identified by their tags. 

A Docker Registry is organized into Docker repositories, where a repository holds all the versions of a specific image. The registry allows Docker users to pull images locally, as well as push new images to the registry. 

DockerHub is the default registry used by the Docker engine, it is the Docker's public registry instance. It is possible to run on-premise the open-source Docker registry/distribution, as well as a commercially supported version called Docker Trusted Registry