i've answered a question like this before so im just going to copy and paste what i said-
Supposedly, it's a group that controls things, and causes mostly bad things. Not much is known about them,though. They're very secretive. I read this a while back, if you have a few hours you should read it. http://www.illuminati-news.com/00363.html
A powerful gang that has supposedly pulled off large attacks on many things it is like a secret fraternity but is not known to exist only rumors i recently heard a lot of stuff about them they seem to be a big thing right now
Illuminati is a group of religious people who believe THEY are the ones with the "direct line" to God. They are a fictitious group based on real-life secret organizations. It is rumored that such a group existed centuries ago.
On 1 May 1776 Weishaupt formed the "Order of Perfectibilists". He adopted the name of "Brother Spartacus" within the order. Though the Order was not egalitarian or democratic, its mission was the abolition of all monarchical governments and state religions in Europe and its colonies. The actual character of the society was an elaborate network of spies and counter-spies. Each isolated cell of initiates reported to a superior, whom they did not know, a party structure that was effectively adopted by some later groups. Weishaupt was initiated into the Masonic Lodge "Theodor zum guten Rath", at Munich in 1777. His project of "illumination, enlightening the understanding by the sun of reason, which will dispel the clouds of superstition and of prejudice" was an unwelcome reform. Soon however he had developed gnostic mysteries of his own, with the goal of "perfecting human" nature through re-education to achieve a communal state with nature, freed of government and organized religion. He began working towards incorporating his system of Illuminism with that of Freemasonry. He wrote: "I did not bring Deism into Bavaria more than into Rome. I found it here, in great vigour, more abounding than in any of the neighboring Protestant States. I am proud to be known to the world as the founder of the Illuminati." Weishaupt's radical rationalism and vocabulary was not likely to succeed. Writings that were intercepted in 1784 were interpreted as seditious, and the Society was banned by the government of Karl Theodor, Elector of Bavaria, in 1784. Weishaupt lost his position at the University of Ingolstadt and fled Bavaria. Activities in exile