Inside Docker Hub (or on a Docker registry you run yourself), images are stored in repositories. You can think of an image repository as being much like a git repository.
It contains images, layers, and metadata about those images. Each repository can contain multiple images (e.g., the Ubuntu repository contains images for Ubuntu 12.04, 12.10, 13.04, 13.10, and 14.04). Each of these images is identified by tags.
For More Information visit the hub site