The basic idea behind Docker is to pack an application with all of its dependencies (let it be binaries, libraries, configuration files, scripts, jars, and so on) into a single, standardized unit for software development and deployment. Docker containers wrap up a piece of software in a complete filesystem that contains everything it needs to run: code, runtime, system tools, and system libraries-anything you can install on a server. This guarantees that it will always run in the same way, no matter what environment it will be deployed in. With Docker, you can build a Node.js or Java project (but you are of course not limited to those two) without having to install Node.js or Java on your host machine. Once you're done with it, you can just destroy the Docker image, and it's as though nothing ever happened. It's not a programming language or a framework; rather, think of it as a tool that helps solve common problems such as installing, distributing, and managing the software. It allows programmers and DevOps to build, ship, and run their code anywhere.

You may think that Docker is a virtualization engine, but it's far from it as we will explain in a while.