Chromium is an open source browser maintained by the Chromium Project which contains the majority of features of the Google Chrome browser. However there are differences including several modules related to Adobe Flash, Widevine DRM, and licensed media codecs supported by Chrome that are not part of Chromium. Use this tag for questions about Chromium specifically and use the google-chrome tag for questions concerning Chrome specifically.
Soon after releasing their Chrome web-browser, Google formed a group to release an open-source version to make it easier and faster for bugs to be fixed and features to be added. (Chromium does not include the auto-update functionality of Chrome.)
While Chromium was originally based on Chrome, now Chrome releases are based on stable Chromium builds. Chromium provides the latest features and bug-fixes (as well as new bugs and regressions) for users who want the bleeding-edge.