CoreOS is a Linux distro based on Gentoo. It is noteworthy for being very small, built for "all-or-nothing" automated updates, and geared for clustering.
Basically if installed on disk (you can also boot it over the network or from a USB stick etc.) it updates by having two system partitions - one "known good" because you've successfully booted from it, and one used to download new updates to, and then will (by default) automatically reboot and switch to update.
The system you end up in is basically systemd + Etcd + Fleet (with Kubernetes on the way) + Docker and rkt, with very little else included.
If you want to spin up a large cluster where everything will run in Docker or rkt containers, that's something CoreOS is well suited for.
Basically if installed on disk (you can also boot it over the network or from a USB stick etc.) it updates by having two system partitions - one "known good" because you've successfully booted from it, and one used to download new updates to, and then will (by default) automatically reboot and switch to update.
The system you end up in is basically systemd + Etcd + Fleet (with Kubernetes on the way) + Docker and rkt, with very little else included.
If you want to spin up a large cluster where everything will run in Docker or rkt containers, that's something CoreOS is well suited for.