terminology

• image: a static snapshot of the ‘hard disk’ of your system (think tar cf image.tar /)
• container: an ephemeral instance of the image (think virtual computer)
• VM or virtual machine: something a container isn't
• registry: a place to store images, DockerHub being the biggest (think GitLab)
• OCI Specifcations: the standard that defines these things (think C++11)
• Docker, podman, Buildah, udocker: implementation of (parts of) this standard (think g++)