Chromium and Chromium OS refer to a pair of open-source projects that power the Google Chrome Web browser and Google Chrome OS, respectively. The Chromium OS project aims to develop a Web-focused operating system and computing experience that's "fast, simple and secure," while the Chromium project's mission is to build an interface for users to experience the Web that's "faster, safer, and more stable than existing alternatives."
The Chromium OS operating system is based on Linux and is designed to run on only specifically designed hardware such as Chromebooks. The Chromium OS offers a user interface that's similar to the Google Chrome browser, and in addition to its heavy focus on the Web experience, the Chromium OS also relies extensively on cloud-based applications.
The Chromium OS project also offers extensive documentation and source code for developers interested in exploring and/or contributing to the project.