To download a particular image, or set of images (i.e., a repository), use docker pull:
$ docker pull debian
# will pull all the images in the debian repository
$ docker pull debian:testing
# will pull only the image named debian:testing and any intermediate layers# it is based on. (Typically the empty `scratch` image, a MAINTAINERs layer,# and the un-tarred base).
$ docker pull registry.hub.docker.com/debian
# manually specifies the path to the default Docker registry. This could# be replaced with the path to a local registry to pull from another source.