What is Docker?

Docker is a platform that runs an application together with everything it needs (libraries, runtime, configuration files) in an isolated package called a container. This removes the "it worked on my machine" problem: the same container behaves the same on the developer's laptop, on the test server and in the cloud.

docker run nginx      # runs the nginx web server with one command

On a machine with Docker installed, the first try is usually the hello-world image. If Docker can't find the image locally it downloads it from Docker Hub, creates a container, runs it, and the container stops once its work is done:

$ docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
Hello from Docker!
This message shows that your installation appears to be working correctly.

This single command does all the steps of downloading, creating and running.