What is Docker?

Think of Docker like a shipping container for software. Just like a real shipping container holds anything (clothes, food, cars) and can be moved between trucks, ships and trains without repacking — a Docker container holds your application (code, libraries, settings) and can be moved between laptops, servers and clouds without any surprises.

Container vs Virtual Machine

Virtual Machine                   Docker Container
+----------------+                +----------------+
| App            |                | App            |
| Libraries      |                | Libraries      |
| Guest OS       |  <-- heavy     +----------------+
| Hypervisor     |                | Docker Engine  |
| Host OS        |                | Host OS        |
+----------------+                +----------------+
GB in size, minutes to boot       MB in size, seconds to boot

A VM has its own operating system. A container shares the host OS kernel — that's why it's small and starts in a second.

The 5 words you must know

Word	Meaning in plain English
Image	A blueprint for a container (like a class in code).
Container	A running copy of that image (like an object).
Dockerfile	A text recipe that describes how to build the image.
Registry	A place where images live (Docker Hub is the public one).
Volume	A folder outside the container where you keep files that must survive restarts.

Your first three commands

# 1. Download and run an image called "nginx"
docker run -d -p 8080:80 nginx

# 2. See what is running
docker ps

# 3. Stop it
docker stop <container-id>

Open http://localhost:8080 and you've just served a webpage from a container.

Real-world example

Imagine you built a Python app that works on your Mac but crashes on your friend's Windows machine. If you wrap it in a Docker image, both of you (and the production server) run the exact same environment. No more "works on my machine".

Where to learn next

• Read the official Docker docs.
• Try building your own image with a Dockerfile (next article in this series).