When Docker engines are running in a cluster, they are said to be in a swarm mode. You can enable the swarm mode for a Docker Engine by initializing a swarm, or by joining an existing swarm.

A swarm is a cluster of Docker engines or nodes where your application services are running. When in swarm mode, the Docker Engine CLI and API include commands to manage the swarm nodes (like add or remove nodes), and deploy and orchestrate the services across the swarm. When you run a docker Docker without using the swarm mode, you are executing container commands directly, and when you run a Docker in the swarm mode, you orchestrate your services.