Chromium is the open source web browser project from which Google Chrome draws its source code. Chromium is essentially similar to Google Chrome, except by the omission of certain Google additions, most noticeable among them: Google's brand, auto-update mechanism, usage-tracking, and bundling of Adobe Flash Player.
Chromium Portable is a custom-built version of the Chromium web browser, optimized for portable use and packaged in the PortableApps.com Format, so you can run it from an iPod, USB flash drive, portable hard drive, CD or any other portable media. You can take your bookmarks, extensions, themes and plugins with you, and Chromium Portable will leave no personal information behind on the machine you run it on.
Chromium Portable is based on the Dev channel of Chromium/Google Chrome, and is updated at least twice per month. You can read more about the Google Chrome release channels here.