Chrome OS is developed by Google. It is a version of Linux designed for one thing – run the Chrome/Chromium web browser. There are a lot of related products that share similar words.
Chrome OS is designed to be useful out of the box and pre-installed on dedicated hardware called Chromebooks. Internally Chrome OS uses the Gentoo Linux portage (wikipedia) package management system. It is a very flexible source code distribution (emerge) and build (ebuild) system.