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Docker packages applications into containers and supplies the tools to build, run, connect, and distribute them. Its pieces have distinct jobs: a Dockerfile describes how to build an image, an image is a template, a container is a running instance, Docker Engine manages Docker objects, and Compose describes how application services fit together.
What Docker is—and what a container is
Docker is a platform for packaging and running applications in containers. A container is a process running on a host with its own filesystem, networking, and isolated process tree. That isolation is useful, but a container is not automatically a separate virtual machine: Docker’s documentation describes containers as isolated processes, rather than complete guest operating systems.
An image is the read-only template used to create a container. The container is the runnable instance, with runtime settings applied. One image can be used to create multiple containers.
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Docker Engine uses a client-server architecture. The docker command-line interface (CLI) is the client. It sends requests through Docker APIs to dockerd, the long-running daemon that performs the work and manages Docker objects such as images, containers, networks, and volumes. The client and daemon may run on the same computer or communicate across machines.
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This distinction helps explain why a command entered in the CLI can affect resources managed elsewhere: the CLI is the interface, while the daemon carries out the requested operation.
How Dockerfiles, images, and containers relate
A Dockerfile contains instructions for building an image. Those instructions produce image layers; the resulting image is read-only. When Docker starts a container from an image, the container gets a writable layer for runtime changes.
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That writable layer is not a substitute for persistent storage. Changes that are not kept in persistent storage disappear when the container is removed. Docker volumes provide storage that is managed separately from a container’s writable layer.
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|---|---|---|
| Dockerfile | Instructions for building an image | When defining how an application image is assembled |
| Image | Read-only template used to create containers | When distributing or starting an application environment |
| Container | Runnable image instance with runtime settings and a writable layer | When the application process is running |
| Compose file | Configuration for an application’s services and related resources | When describing how multiple parts of an application run together |
What Docker Compose adds
Docker Compose is a tool for defining and managing applications made up of multiple containers. A Compose file, usually named compose.yaml, describes services and can define the networks and volumes those services need. A service represents an application component and can be implemented by one or more containers created from the same image and configuration. A Compose project groups the resources for an application.
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Services communicate through networks. For example, an application may have a web-facing service and a separate data service, with Compose configuration describing both and how they connect. Compose expresses the application-level arrangement; Docker Engine remains the machinery that manages the underlying Docker objects.
Docker Engine versus Compose
Engine and Compose are related, but they operate at different levels. Engine provides the client, API, and daemon used to manage Docker objects. Compose describes a set of application services and their connections, then uses Docker tooling to work with that application.
Keep the two configuration files distinct: a Dockerfile tells Docker how to build an image; a Compose file describes services and their runtime configuration. Docker’s guidance is that “each container should do one thing and do it well,” while noting that exceptions exist. Treat this as a useful design principle, not a rule that every application must follow without exception.
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Registries store Docker images so they can be shared and retrieved. Docker Hub is a public registry, and Docker uses it by default unless configured to use another registry. The image is the distributable template; containers are created from it where the application is run.
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Choosing a Compose installation route
Docker’s installation overview identifies Docker Desktop as the recommended and easiest way to get Compose across Linux, macOS, and Windows. On Linux systems where Docker Engine and the CLI are installed, Compose is also available as a plugin. Docker describes the standalone Compose installation route as legacy and does not recommend it.
Exact setup steps and licensing terms can vary by operating system and change over time. Follow the current operating-system-specific instructions in the Docker Compose installation guide rather than relying on generic commands.
A practical mental model
- Describe the build: put image-building instructions in a Dockerfile.
- Create the template: build an image from those instructions.
- Run the application process: start a container from the image, with runtime settings.
- Connect application components: describe services, networks, and any persistent volumes in a Compose file when the application has multiple parts.
- Let Engine manage Docker objects: the CLI sends requests, and the daemon performs the work.
These roles provide a useful way to understand Docker without treating its terms as interchangeable. For an overview of the platform, see Docker’s overview; for the client-server details, see Docker Engine documentation. Compose’s application model and service concepts are described in the Compose overview and service reference.
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