IT/Software/Virtualization/Containerization/Docker
Docker runs a Linux OS virtual image and then adds whatever you want on top of it. It can be used simply as a command line only linux environment for any OS, or as a way to run software separate from the host OS. It is supposed to be a simple way to spin up any software quickly and have it isolated from the host OS.
Examples
Start a simple Ubuntu container:
sudo docker run -it ubuntu