Footloose creates containers that look like virtual machines. Those containers run systemd as PID 1 and a ssh daemon that can be used to login into the container. Such "machines" behave very much like a VM, it's even possible to run dockerd in them :)
Footloose can be used for a variety of tasks, wherever you'd like virtual machines but want fast boot times or need many of them. An easy way to think about it is: Vagrant, but with containers.
In this talk Damien Lespiau, Software Engineer at Weaveworks and the creator of footloose, will walk you through installation, usage, example use-cases, and answer your questions.