Docker is an open platform for developing, shipping and running applications, made by Docker Inc. It packages an application into a container: an isolated environment that holds the program and everything it needs, so it works the same way in a local data center or with a cloud provider. A container starts from an image, a read-only template with the instructions for creating the container.
Docker uses a client-server design. The docker command-line client sends commands such as docker run to the Docker daemon (dockerd), which builds, runs and distributes containers. Images are kept in registries: Docker Hub is a public registry that anyone can use, and Docker pulls images from it by default, so a project can publish its software there as an image for others to download. Docker Desktop bundles the daemon, the client, Docker Compose (for applications made of several containers) and Kubernetes for Mac, Windows and Linux. Newer products add isolated sandboxes for AI coding agents and hardened images.
Docker Personal is free for individual developers; Pro, Team and Business are subscriptions billed per user.