Chrome OS is Google's version of Linux, based on Google's Chrome browser. Chrome OS requires a constant Internet connection for most apps. Crouton is a method to run GNU/Linux on Google Chromebooks.
The ARM-based Chromebooks ship with Das U-Boot. The Intel x86-based Chromebooks run it on top of Coreboot. Coreboot is the evolution of LinuxBIOS.
Chrome OS uses Gentoo's package management system, Portage for application installation. Portage, written in Python and Bash, are not pre-compiled. They compile on-site, allowing for download of hardware drivers and user chosen features.
New Chromebooks use a new display-backend, known as Freon. With Freon, the Chrome browser talks directly to the kernel's APIs and is also communicating directly with OpenGL ES for the 3D drawing functionalities.
The Open source only version of "Chrome OS" is called "Chromium OS". That is where all the development takes place.