I love that CoreOS uses lean-startup style voting for biz-dev type decisions (e.g. which clouds should we run on next?); yet can also produce new technologies like etcd, fleet & Rudder through "vision" - knowing which pieces are missing.
It's not an easy combination to pull off, and it's building to a really great system that seems to be part of all the interesting new developments (Kubernetes, DO, Panamax.io etc)
1. Docker. Essentially, you can make a container around your Rails app and launch it anywhere. Docker isn't exclusive to CoreOS, but CoreOS happens to be really good at it.
2. fleet. You specify a configuration file when you launch a CoreOS droplet with DigitalOcean, and it teaches the system how to launch your docker container automatically, and coordinate with any other CoreOS droplets you have running.
This means you can teach your machines to, say, not all reboot for updates at the same time and thus bring down your service. They'll argue amongst themselves about who reboots when.
It also means that if you tell certain instances to identify as DB instances, and certain instances to identify as Rails instances, they'll know how to find each other, and you can teach them to fail over to each other if you have redundancy.
Docker is a technology that lets a VM use resources of the host (threads, etc) directly, so you get VMs that are super super light weight. With Docker, even your laptop can run 100 VMs, all with different OSes and customizations (though linux only) without breaking a sweat.
CoreOS is a special linux flavor designed to be used specifically in this role, making the VMs even MORE light weight (and has features to coordinate multiple VMs working together.)
DigitalOcean allows you to host VMs in the cloud. Now you have the full shebang: A hosting provider designed to run an OS designed to run efficiently as a VM.
It's not an easy combination to pull off, and it's building to a really great system that seems to be part of all the interesting new developments (Kubernetes, DO, Panamax.io etc)