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.
After Google acquired Android in 2005 another group at Google announced Chrome OS in 2009 as a single purpose system designed to run the Google Chrome browser first released in 2008.
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.