I haven't really looked into it, but I get the idea that Google wrote Chrome, then released most of it under an open source license and distributed that as Chromium. Now, development on Chromium by volunteers plus Google employees is pulled into Chrome and a few things are added to it by Google.
The Chromium project doesn't actually distribute binaries (compiled executables), just the source code, so, while I'm sure builds for commercial OSs exist, it's only really free OSs that use it. It's Chrome without the branding, update mechanism (free OSs tend to have package managers), tracking, Flash (to keep it free) (bundled, that is - you just need to install Flash separately like with most other browsers) and a load of licensing terms Chrome needs you to agree to.