Docker Terminology

To learn Docker effectively, you first need to understand its unique language. The names and concepts can be confusing at first, but they all fit together in a logical way.

The 3 Pillars of Docker

1. The Docker Image (The Blueprint)

An image is a read-only template that contains everything your application needs to run: the code, libraries, settings, and the operating system.

Analogy: It’s like a recipe for a cake. You can’t eat the recipe, but you use it to bake the cake.

2. The Docker Container (The Actual Application)

A container is a running instance of an image. You can have many containers running from the same image at the same time.

Analogy: It’s the actual cake baked from the recipe.

3. The Docker Registry (The Library)

A registry is a place where Docker images are stored so you can share them with others or download images that others have created.

Analogy: It’s like a cookbook (a collection of recipes). The most popular registry is Docker Hub.

Relationship Diagram

       [ DOCKER REGISTRY ] (Library of blueprints)
               ↓ (Pull)
       [ DOCKER IMAGE ] (A single blueprint)
               ↓ (Run)
       [ DOCKER CONTAINER ] (A running instance)

Other Important Terms

Docker Hub

The default, public registry where most developers find official images for tools like Node.js, Python, PostgreSQL, and more.

Dockerfile

A simple text file with instructions on how to build a Docker image. (You’ll learn how to write these soon!)

Volume

A way to store data outside of a container so it doesn’t disappear when the container stops.

Docker Compose

A tool for defining and running multi-container applications (e.g., an API + a Database).

How Junior Developers Encounter This

“I need to pull the Postgres image”: (Getting the blueprint from the registry).
“Is the container running?”: (Checking if the application instance is active).
“We need to update the Dockerfile”: (Changing the instructions for the blueprint).

Common Beginner Mistakes

Confusing Images and Containers: Remember: you “build” an image, and you “run” a container.
Thinking Containers are Virtual Machines (VMs): While they look similar, containers are much smaller and faster because they share the computer’s operating system.
Thinking a Registry is a Database: Registries store images (blueprints), not the actual data your application produces.

Next Lesson

Now let’s see these blueprints and instances in action:

➡ Images and Containers