Swarm is described by Docker itself as:
Docker Swarm is native clustering for Docker. It turns a pool of Docker hosts into a single, virtual Docker host.
Swarm is a tool that gives you the illusion to manage a single-huge Docker host made of many Docker hosts, as they were one and had one command-entry point. It allows you to orchestrate and operate a certain number of containers on these hosts, using the routine Docker tools, either using the Docker native or the python-docker client, even curl with the Docker Remote APIs.
This is what a minimal Swarm cluster in production looks similar to:
