Google's Chrome OS is an open source, lightweight operating system that will initially be targeted at x86 and ARM based netbooks. Chrome OS is a new project, separate from Android and was initially announced in July 2009. Like with Chromium and Chrome, ChromiumOS is the open source release of ChromeOs.
ChromeOS is currently still under development, but Google has released its first version, together with a development platform called CR48. You can get your hands on one of these through the pilot program.
There is some quite extensive documentation about working on and with ChromeOs on the ChromiumOS project page