CoreOS is just that: a core Linux operating system. Designed as a minimal Linux, it is free of all the packages that come pre-installed with standard distributions. The reason for that is because, once the OS has booted up, CoreOS has been designed for running docker containers.
CoreOS has some unique features, like:
some sort of dual-boot feature for auto-updates, where one instance of the OS updates the main installation
fleet, a cluster management tool that promises to be like systems for clusters (developed in Go)
and etcd, a distributed key-value database (also developed in Go), as the main configuration system.