Table of Contents
A New Way to Package and Run Applications
Docker is a platform that lets you package an application together with everything it needs to run, and then run that package the same way on any computer that supports Docker. At its core, Docker helps you solve a simple but painful problem in software development: an application often works on one machine but fails on another because the environment is different.
With Docker, you put your application, its libraries, and its runtime into a container. The container is a self-contained, isolated environment that behaves like a tiny, dedicated system for your app. You can start, stop, copy, and move containers easily, which makes applications much more portable and predictable across different machines.
Containers and the Idea of Isolation
A container is a running instance of an image. You can think of it as a lightweight, isolated box that holds a process and everything that process needs. Unlike traditional virtual machines, containers share the host operating system kernel but keep applications separated from each other. This isolation means that different applications, with different versions of tools or libraries, can run side by side on the same machine without interfering.
Docker uses this isolation to make environments reproducible. If a container runs correctly on your laptop, the same container will behave the same way on another laptop or on a server, as long as Docker is available.
Images as Blueprints
Before you can run a container, you need an image. An image is a read-only template that describes what a container should look like. It contains the application, its dependencies, and configuration defaults, prepared in advance. When you start an image, Docker creates a container from it.
Images are built in layers, which are stacked on top of each other. Each layer usually represents a change, such as installing a package or adding a file. This layered structure lets Docker reuse parts of images across different containers, which saves space and speeds up downloads and builds.
The Docker Engine
Docker is powered by the Docker Engine. This is the core software that runs on your machine and knows how to build images, store them, and start or stop containers. You interact with the Docker Engine using commands such as docker run or docker build, usually from a terminal or through integration in tools and platforms.
When you issue a Docker command, Docker Engine decides what images to pull, what containers to create, how to isolate them, and how to connect them to the network or to your local files.
Portability Across Environments
A key promise of Docker is that you can run the same container image in many environments. This includes laptops, on-premise servers, cloud virtual machines, and managed container platforms. The image does not need to be changed for each environment, so you avoid repeating manual setup steps or maintaining separate instructions.
Because of this, Docker is frequently described as a tool that turns applications into portable units. You can think of shipping an image to another machine as sending a ready-to-run package, instead of a list of installation instructions.
Reproducible and Shareable Environments
Docker also works as a way to standardize environments for development and testing. A team can agree on a particular image for a database or a programming language runtime. Each developer then uses the same image on their own machine, which reduces surprises when code is shared or merged.
Images can be published and shared through container registries. Public images often provide standard building blocks, such as base operating systems or language runtimes, which you can reuse in your own images.
Key idea: A Docker image is a portable, shareable template. A Docker container is a running, isolated instance of that image.
A Tool, Not a Language or Framework
Docker is not a programming language, a web framework, or a database. It does not replace these tools. Instead, it wraps them. You still build your application using your usual tools. Docker comes into play when you package and run that application.
By separating how you build an application from how you package and run it, Docker lets you keep your existing technology choices while gaining better consistency and portability for your runtime environment.