What is Docker?
Docker is a containerisation software which enables you to develop apps and run them in an environment that is contained. It is an alternative to virtual machines.
Images & Layers
Images represent how a docker container is built in a standardised way. They are immutable, just like a picture. You can add more components to a container using layers. Each RUN command adds a new layer to the image. Layers represent slight changes that are added to each image.