In Docker terminology, a read-only Layer is called an image. An image never changes.
Since Docker uses a Union File System, the processes think the whole file system is mounted read-write. But all the changes go to the top-most writeable layer, and underneath, the original file in the read-only image is unchanged. Since images don't change, images do not have state.
Each image may depend on one more image which forms the layer beneath it. We sometimes say that the lower image is the parent of the upper image.
An image that has no parent is a base image.
All images are identified by a 64 hexadecimal digit string (internally a 256bit value). To simplify their use, a short ID of the first 12 characters can be used on the command line. There is a small possibility of short id collisions, so the docker server will always return the long ID.