Chromebooks run an official version of Google's Chrome OS. Initially they were more-or-less like a lower powered netbook, & instead of running a version of Windows, pretty much everything was done online using the browser. Now newer models are starting to be able to run Android apps from the Play store. Some rumors say that Google will merge Chrome and Android -- other rumors say that a new OS Google's working on will replace Chrome.
Chromium is the open source version of the Chrome OS, and Neverware is a company that put together a free version designed for older hardware, e.g. older laptops in schools. While they continue to develop that version, they've added a few more, including an Office 365 friendly version developed in partnership with Microsoft.
neverware[.]com/office365#office365title
neverware[.]com/pricing
wikipedia[.]org/wiki/Chromium_OS