How to use Docker ?

First of all , what is docker ?... I came across docker when I was searching for code editor and how to compile C code form web applications.
http://qnimate.com/create-an-frontend-editor-with-code-highlighting-and-execution/

 Docker - https://www.docker.com/

Docker is an open-source project that automates the deployment of applications inside software containers, by providing an additional layer of abstraction and automation of operating-system-level virtualization on Linux. https://en.wikipedia.org/wiki/Docker_(software)

I actually want to run the code in a virtual machine ( for this idea http://naveendavisv.blogspot.com/2015/08/thinking-to-build-microcontroller.html) and show the output in a text area, even though I don't need the full output, but if  the code can be checked for syntax error, I am much happy...