Docker is a tool that packages an application together with everything it needs to run into a single unit called a container, so it runs the same way on every machine. Let's unpack what that means and why it's so useful.
The problem Docker solves
You've probably heard the phrase "but it works on my machine". An app runs fine on your laptop, then breaks on a colleague's computer or on the server. Usually the reason is the same: the two machines have different versions of a language, a library, or some system tool.
Docker solves this by packaging your application together with everything it needs to run - the code, the runtime, the libraries, the settings - into a single unit called a container. That container runs the same way everywhere: your laptop, a coworker's machine, or a production server.
What is a container?
A container is an isolated process running on your computer. Think of it as a lightweight, self-contained box that holds one application and everything that application needs. Inside the box, the app believes it has its own operating system, its own files, and its own network - but it's actually sharing your computer's resources with other containers.
Because each container is isolated, you can run many of them side by side without them interfering with each other. One container can use one version of a tool while another uses a completely different version, and neither one knows about the other.
What is Docker, then?
Docker is the tool that builds, runs and manages these containers. You describe what your app needs, Docker packages it up, and then you can run that package anywhere Docker is installed.
In the next lessons we'll break these ideas down further and actually run something. For now, remember the one-sentence version:
Docker packages an application with everything it needs so it runs the same way everywhere.
We'll define the exact difference between a container and an image in the images and containers lesson
- for now, "container = a running, isolated box with your app inside" is all you need.
A common misconception to clear up
A lot of people meet Docker expecting it to make their app faster. It usually doesn't. Docker's real job is consistency, not speed: the same environment on every machine, so "works on my machine" stops being a problem. It's also not a virtual machine (that's the next lesson). Keep that framing and Docker will make a lot more sense as you go.
FAQ
Is Docker free to use?
Yes. Docker Engine is open source and free, and Docker Desktop is free for personal use, education, and small businesses. You can follow this entire course without paying.
Do I need to know Linux to use Docker?
Not to get started. A little comfort with the terminal helps, and most images are Linux-based, but you can run Docker on Windows or macOS and pick up the Linux parts as you need them.
Is Docker a programming language?
No. Docker is a tool for packaging and running applications. You use it alongside whatever language your app is written in - PHP, JavaScript, Python, and so on.