What is Moby?
Moby is an open framework created by Docker to assemble specialized container systems without reinventing the wheel. It provides a “lego set” of dozens of standard components and a framework for assembling them into custom platforms. For further information you can check the Moby site which better describes the different components involved
About Play with Moby
Play-with-moby (PWM) is a site made by Docker captains Marcos Nils and Jonathan Leibiusky as an extension of PWD. PWM is a Moby playground which allows you to try different components of the platform in seconds. It gives you the experience of having a free Alpine Linux Virtual Machine in the cloud where you can build and run Moby projects and even create clusters to experiment. Under the hood DIND or Docker-in-Docker is used to give the effect of multiple VMs/PCs.