Difference between Running and Starting a Docker container

Explanation with an example:

Consider you have a game (iso) image in your computer.

When you run (mount your image as a virtual drive), a virtual drive is created with all the game contents in the virtual drive and the game installation file is automatically launched. [Running your docker image - creating a container and then starting it.]

But when you stop (similar to docker stop) it, the virtual drive still exists but stopping all the processes. [As the container exists till it is not deleted]

And when you do start (similar to docker start), from the virtual drive the games files start its execution. [starting the existing container]

In this example - The game image is your Docker image and virtual drive is your container.

This is a very important question and the answer is very simple, but fundamental:

1. Run: create a new container of an image, and execute the container. You can create N clones of the same image. The command is: docker run IMAGE_ID and not docker run CONTAINER_ID

2. Start: Launch a container previously stopped. For example, if you had stopped a database with the command docker stop CONTAINER_ID, you can relaunch the same container with the command docker start CONTAINER_ID, and the data and settings will be the same.

• run runs an image
• start starts a container.

The docker run doc does mention:

The docker run command first creates a writeable container layer over the specified image, and then starts it using the specified command.

That is, docker run is equivalent to the API /containers/create then /containers/(id)/start.

You do not run an existing container, you docker exec to it (since docker 1.3).
You can restart an exited container.