CoreOS is designed for security, consistency, and reliability. Instead of installing packages via yum or apt, CoreOS uses Linux containers to manage your services at a higher level of abstraction. A single service's code and all dependencies are packaged within a container that can be run on one or many CoreOS machines.
Linux containers provide similar benefits as complete virtual machines, but focus on applications instead of entire virtualized hosts. Since containers don’t run their own Linux kernel or require a hypervisor, they have almost no performance overhead. The lack of overhead allows you to gain density which means fewer machines to operate and a lower compute spend.
CoreOS runs on almost any platform, including Vagrant, Amazon EC2, QEMU/KVM, VMware and OpenStack and your own hardware. If you're currently running in the cloud, running a single CoreOS cluster on two different clouds or cloud + bare metal is supported and encouraged.