It helps if one knows what the real illuminati was.. from wiki:
The Illuminati (plural of Latin illuminatus, "enlightened") is a name given to several groups, both real and fictitious. Historically, the name refers to the Bavarian Illuminati, an Enlightenment-era secret society founded on May 1, 1776. The society's goals were to oppose superstition, obscurantism, religious influence over public life and abuses of state power. "The order of the day," they wrote in their general statutes, "is to put an end to the machinations of the purveyors of injustice, to control them without dominating them." The Illuminati—along with Freemasonry and other secret societies—were outlawed through Edict, by the Bavarian ruler, Charles Theodore, with the encouragement of the Roman Catholic Church, in 1784, 1785, 1787 and 1790. In the several years following, the group was vilified by conservative and religious critics who claimed that they continued underground and were responsible for the French Revolution.
The Owl of Minerva perched on a book was an emblem used by the Bavarian Illuminati in their "Minerval" degree.
Adam Weishaupt (1748-1830) was a professor of Canon Law and Practical philosophy at the University of Ingolstadt. He was the only non-clerical professor at an institution run by Jesuits, whose order had been dissolved in 1773. The Jesuits of Ingolstadt, however, still retained the purse strings and some power at the University, which they continued to regard as their own. Constant attempts were made to frustrate and discredit non-clerical staff, especially when course material contained anything they regarded as liberal or Protestant. Weishaupt became deeply anti-clerical, resolving to spread the ideals of the Enlightenment (Aufklärung) through some sort of secret society of like-minded individuals.
Finding Freemasonry to be expensive, and not open to his ideas, he founded his own society which was to have a gradal system based on Freemasonry, but his own agenda. His original name for the new order was Bund der Perfektibilisten, or Covenant of Perfectibility, but quickly changed it because it sounded too strange. On 1 May 1776 Weishaupt and four students formed the Illuminatenorden, or Order of Illuminati, taking the Owl of Minerva as their symbol. The members were to use aliases within the society. Weishaupt became Spartacus. Law students Massenhausen, Bauhof, Merz and Sutor became respectively Ajax, Agathon, Tiberius and Erasmus Roterodamus. Weishaupt later expelled Sutor for indolence.
Massenhausen was initially the most active in expanding the society. Significantly, while studying in Munich shortly after the formation of the order, he recruited Xavier von Zwack, a former pupil of Weishaupt at the beginning of a significant administrative career. (At the time, he was in charge of the Bavarian National Lottery). Massenhausen's enthusiasm soon became a liability in the eyes of Weishaupt, often attempting to recruit unsuitable candidates. Later, his erratic love-life made him neglectful, and as Weishaupt passed control of the Munich group to Zwack, it became clear that Massenhausen had misappropriated subscriptions and intercepted correspondence between Weishaupt and Zwack. In 1778, Massenhausen graduated and took a post outside Bavaria, taking no further interest in the order. At this time, the order had a nominal membership of twelve.
With the departure of Massenhausen, Zwack immediately applied himself to recruiting more mature and important recruits. Most prized by Weishaupt was Hertel, a childhood friend and a canon of the Munich Frauenkirche. By the end of summer 1778 the order had 27 members (still counting Massenhausen) in 5 commands; Munich (Athens), Ingolstadt (Eleusis), Ravensberg (Sparta), Freysingen (Thebes), and Eichstaedt (Erzurum).
During this early period, the order had three grades of Novice, Minerval, and Illuminated Minerval, of which only the Minerval grade involved a complicated ceremony. In this the candidate was given secret signs and a password. A system of mutual espionage kept Weishaupt informed of the activities and character of all his members, his favourites becoming members of the ruling council, or Areopagus. Some novices were permitted to recruit, becoming Insinuants. Christians of good character were expected, with Jews and Pagans specifically excluded, along with women, monks, and members of other secret societies. Favoured candidates were rich, docile, willing to learn, and aged 18-30.
Having, with difficulty, dissuaded some of his members from joining the Freemasons, Weishaupt decided to join the older order to acquire material to expand his own ritual. He was admitted to lodge "Prudence" of the Rite of Strict Observance early in February 1777. His progress through the three degrees of "blue lodge" masonry taught him nothing of the higher degrees he sought to exploit, but in the following year a priest called Abbé Marotti informed Zwack that these inner secrets rested on knowledge of the older religion and the primitive church. Zwack persuaded Weishaupt that their own order should enter into friendly relations with Freemasonry, and obtain the dispensation to set up their own lodge. At this stage (December 1778), the addition of the first three degrees of Freemasonry was seen as a secondary project.
With little difficulty, a warrant was obtained from the Grand Lodge of Prussia called the Royal York for Friendship, and the new lodge was called Theodore of the Good Council, with the intention of flattering Charles Theodore, Elector of Bavaria. It was founded in Munich on 21 March 1779, and quickly packed with Illuminati. The first master, a man called Radl, was persuaded to return home to Baden, and by July Weishaupt's order ran the lodge.
The next step involved independence from their Grand Lodge. By establishing masonic relations with the Union lodge in Frankfurt, affiliated to the Premier Grand Lodge of England, lodge Theodore became independently recognised, and able to declare its independence. As a new mother lodge, it could now spawn lodges of its own. The recruiting drive amongst the Frankfurt masons also obtained the allegiance of Adolph Freiherr Knigge.
Expansion and persecution
Fundamental changes occurred in the wake of the acceptance of Adolph Freiherr Knigge into the order. Knigge was a young author and Freemason who was steeped in the Western mystery traditions from an early age. On his admission to the order, in 1780, he was charged with recruiting in five areas in the German states, and was soon in charge of hundreds of pupils. Asking for admission to the higher degrees of the order, to teach them to his recruits, he discovered that these degrees only existed in Weishaupt's mind. He was obliged (with Weishaupt's full authority) to write them himself. When Weishaupt attempted to retroactively edit Knigge's work, the two men fell out. Knigge left the order in 1783, depriving Weishaupt of his best theoretician, recruiter, and apologist.
In 1777, Karl Theodor became ruler of Bavaria. He was a proponent of Enlightened Despotism and his government banned all secret societies including the Illuminati. Internal rupture and panic over succession preceded the society's downfall. A government edict dated March 2, 1785 "seems to have been deathblow to the Illuminati in Bavaria". Weishaupt had fled and documents and internal correspondence, seized in 1786 and 1787, were subsequently published by the government in 1787. Von Zwack's home was searched and much of the group's literature was disclosed.
Many influential intellectuals and progressive politicians counted themselves as members, including Ferdinand of Brunswick and the diplomat Xavier von Zwack, who was the Order's second-in-command. It attracted literary men such as Johann Wolfgang von Goethe and Johann Gottfried Herder and the reigning dukes of Gotha and Weimar.
Barruel and Robison
Between 1797 and 1798, Augustin Barruel's Memoirs Illustrating the History of Jacobinism and John Robison's Proofs of a Conspiracy publicised the theory that the Illuminati had survived and represented an ongoing international conspiracy. This included the claim that it was behind the French Revolution. Both books proved to be very popular, spurring reprints and paraphrases by others. A prime example of this is Proofs of the Real Existence, and Dangerous Tendency, Of Illuminism by Reverend Seth Payson, published in 1802. Some of the response to this was critical, for example Jean-Joseph Mounier's On the Influence Attributed to Philosophers, Free-Masons, and to the Illuminati on the Revolution of France.
The works of Robison and Barruel made their way to the United States, and across New England, Reverend Jedidiah Morse and others gave sermons against the Illuminati. Their sermons were printed and the matter was followed in newspapers. Concern died down in the first decade of the 1800s, although it revived from time to time in the Anti-Masonic movement of the 1820s and 30s.
Pretty much everything else you've heard is hollywood making use of it for a plot device.. nothing like a good conspiracy about secret organizations.