RancherOS is a tiny Linux distro that runs the entire OS as Docker containers.
RancherOS is a minimalist Linux distribution perfect for running Docker containers. By running Docker directly on top of the kernel and delivering Linux services as containers, RancherOS delivers just what you need to build what you want.
An OS made of Containers
Everything in RancherOS is a container. One instance of Docker runs directly on the kernel, replacing the traditional init system, and system services run within this System Docker. User containers run inside a separate daemon we call User Docker.
Traditional Linux distributions have to be manually tracked by sysadmins. RancherOS is designed from the ground up to leverage the Docker API and host sophisticated management agents. Teams can use the Rancher container management platform to easily manage RancherOS at large scale in production.
User containers run in a separate Docker daemon
RancherOS creates a special system service container called User Docker, that is created by System Docker. A separate Docker daemon runs in the User Docker container. Because all user containers run inside the User Docker container, deleting all user containers, for example, will not bring down the system containers running RancherOS services.
RancherOS is 20mb of just what is needed to run Docker
RancherOS is a remarkably tiny operating system, running everything you need to run your containers and nothing more. By keeping RancherOS so small, we minimize the need for frequent patching, and free up space on your hosts.
Simple Updates and Rollbacks
RancherOS leverages the extremely powerful Docker packaging and distribution support to deliver updates and features of the operating system. All system services are delivered as Docker containers, and while the kernel and initial ram disk are not Docker containers, we use Docker packaging and distribution to deliver kernel and ram disk update as well.
Also See: http://rancher.com/comparing-rancher-orchestration-engine-options/