Chromium is not only the name of a browser, but also of the open-source project that generates the source code used by Chrome. Google is the primary backer of Chromium – it kicked off the project when it launched Chrome in September 2008 – but because the code is open-source, others, including people not employed by Google, contribute to the Chromium project. (Microsoft, for one, started serious input this year; see the “And now Microsoft’s Edge?” section below for more information.)
The browser compiled from the current Chromium source code is called not surprisingly, Chromium. Chrome, on the other hand, begins with Chromium but does not end with it. Instead, Google adds proprietary code to Chromium, either its own, like the browser’s automated update mechanism, or someone else’s, such as Adobe Flash (for now at least), to create Chrome.