> For the complete documentation index, see [llms.txt](https://kks32-courses.gitbook.io/hpc-containers/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://kks32-courses.gitbook.io/hpc-containers/docker/fundamentals.md).

# Fundamentals

Now that we’ve got the big picture in place, let’s go through the fundamental parts of Docker piece by piece:

![Docker concepts](https://3073326825-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-L9PtzjOz4HPAc9i-Kmp%2F-L9PyelTER1mFvQFAeb1%2F-L9PygCZIxnh7vrUe604%2Fdocker-concepts.png?generation=1523018550224006\&alt=media)

## Docker Engine

Docker engine is the layer on which Docker runs. It’s a lightweight runtime and tooling that manages containers, images, builds, and more. It runs natively on Linux systems and is made up of:

1. A Docker Daemon that runs in the host computer.
2. A Docker Client that then communicates with the Docker Daemon to execute commands.
3. A REST API for interacting with the Docker Daemon remotely.

## Docker Client

The Docker Client is what you, as the end-user of Docker, communicate with. Think of it as the UI for Docker. For example, when you do `docker build .` you are communicating to the Docker Client, which then communicates your instructions to the Docker Daemon.

## Docker Daemon

The Docker daemon is what actually executes commands sent to the Docker Client — like building, running, and distributing your containers. The Docker Daemon runs on the host machine, but as a user, you never communicate directly with the Daemon. The Docker Client can run on the host machine as well, but it’s not required to. It can run on a different machine and communicate with the Docker Daemon that’s running on the host machine.

## Images and containers

A container is launched by running an image. An *image* is an executable package that includes everything needed to run an application--the code, a runtime, libraries, environment variables, and configuration files.

A *container* is a runtime instance of an image--what the image becomes in memory when executed (that is, an image with state, or a user process). You can see a list of your running containers with the command, docker ps, just as you would in Linux.

Docker containers are built off Docker images. Since images are read-only, Docker adds a read-write file system over the read-only file system of the image to create a container.

![Docker container layers](https://3073326825-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-L9PtzjOz4HPAc9i-Kmp%2F-L9R-LOcloafc2Ytg16q%2F-L9R-MiqJJddK_Ozeys5%2Fcontainer-layer.png?generation=1523035768196676\&alt=media)