Someone's muddled the chrome-related tags!
Chromium is an open-source web browser. Chrome is a closed-source derivative of Chromium made by Google. Chrome OS is an operating system based on the same closed Google browser. Chromium OS, however, is the operating system based directly on the open-source Chromium.
Google's Chrome OS is ONLY available when it comes pre-installed on a Chromebook.
The "Chrome OS" that people try to install on their previously-bought laptops (nightly builds, etc.) is not Chrome OS. It is actually Chromium OS. This fosters many more questions about compiling, installing, etc. than the pre-built, pre-installed, pre-configured Chrome OS.