Here is a link showing the difference between Chrome and Iron. http://www.srware.net/en/software_srware_iron_chrome_vs_iron.php
Read about chromium from this search link. http://www.google.com/search?sourceid=chrome&ie=UTF-8&q=Chromium
In short form, chromium is the base of the Google Chrome browser. Iron uses the chromium base but removes all of the Google crap that has the potential to call home and give out information about your browsing habits.