dockerd – The Docker daemon itself. The highest level component in your list and also the only ‘Docker’ product listed. Provides all the nice UX features of Docker.
(docker-)containerd – Also a daemon, listening on a Unix socket, exposes gRPC endpoints. Handles all the low-level container management tasks, storage, image distribution, network attachment, etc…
(docker-)containerd-ctr – A lightweight CLI to directly communicate with containerd. Think of it as how ‘docker’ is to ‘dockerd’.
(docker-)runc – A lightweight binary for actually running containers. Deals with the low-level interfacing with Linux capabilities like cgroups, namespaces, etc…
(docker-)containerd-shim – After runC actually runs the container, it exits (allowing us to not have any long-running processes responsible for our containers). The shim is the component which sits between containerd and runc to facilitate this.