At first, I was confused too. So I asked on the ML. See [1] and continuation in [2]. You could also browse the docker-user list, there are a number of threads asking to clarify various concepts about Docker.

[1]: https://groups.google.com/d/msg/docker-dev/OcpLrYti64Q/Ue6vP... [2]: https://groups.google.com/d/msg/docker-user/4acVRnOd9Dw/RZRc...