A container is run from a container image.
A container image is a static version of all the files, environment variables, and the default command/program that should be present in a container. Static here means that the container image is not running, it's not being executed, it's only the packaged files and metadata.
In contrast to a "container image" that is the stored static contents, a "container" normally refers to the running instance, the thing that is being executed.
When the container is started and running (started from a container image) it could create or change files, environment variables, etc. Those changes will exist only in that container, but would not persist in the underlying container image (would not be saved to disk).
A container image is comparable to the program file and contents, e.g. python and some file main.py.
And the container itself (in contrast to the container image) is the actual running instance of the image, comparable to a process. In fact, a container is running only when it has a process running (and normally it's only a single process). The container stops when there's no process running in it.
A container image normally includes in its metadata the default program or command that should berun when the container is started and the parameters to be passed to that program. Very similar to what would be if it was in the command line.
When a container is started, it will run that command/program (although you can override it and make it run a different command/program).
A container is running as long as the main process (command or program) is running.
A container normally has a single process, but it's also possible to start subprocesses from the main process, and that way you will have multiple processes in the same container.
But it's not possible to have a running container without at least one running process. If the main process stops, the container stops.
references:
https://fastapi.tiangolo.com/deployment/docker/#what-is-a-container-image
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