The official difference is that the container is the last layer which is writable whereas the layers below are only readable and they belong to your image. The intuitive difference is that the docker instance is the instance virtualized by your docker daemon and the running your image, it operates within an isolated section of your kernel (this process is hidden from you). The image however is static, it doesn't run, it is just a pile of layers (static files). If we would relate this paradigm to object-oriented programming, the image is your class definition, whereas your docker instance is your class spawned object that resides in memory.
I have written a tutorial to reinforce your docker knowledge intuition:
http://javagoogleappspot.blogspot.com/2018/07/docker-basics.html
