[StavrosK]

High-level overview: You get an image (an actual filesystem image). You "docker run" some command, which creates a container (container is to image what VM is to, well, disk image, the container is what runs). Any change to this container doesn't change the base image, but it's done on a separate one. You can commit at any point and "freeze" the container's image and get back to it. You can start and stop the container at any time, but you can only run one starting process in it (so you might want to run sshd or supervisord).

That's pretty much the gist. It's a VM where snapshots are free, you can clone it at any point and branch off, and it's really lightweight to run. Basically, think of it as the child of VirtualBox and git.