The illuminati is a secret society that was founded during the enlightenment era and is believed to be controlling world affairs using different means. In modern times they are believed to a group who are set on bringing a New World Order which will control all humans on earth. See http://en.wikipedia.org/wiki/Illuminati for more information.
They were originally founded so like minded individuals could gather In secret to discuss things such as astronomy, philosophy and many other things that at the time was considered blasphemy by the church. Others are saying things such as New World Order and that may be true. But men like George Washington and Ben Franklin, Free Masons, succeeded in accomplishing that New World Order. It's called the United States of America.