The Illuminati (plural of Latin illuminatus, meaning 'enlightened') is the name by which several groups, sects, occult organizations and shadowly societies are known throughout human history and lore. The first known use of the term Illuminati was 15th-century Europe, being associated with various occult sects and secret orders, including the Rosicrucians and the Freemasons. Because of the secrecy, it's impossible to point out a WHO. However, the clandestine group is often cited as the collective of the elite powers who control the world. Those whom have the most knowledge of the true goals of each society. Having a hand in everything, their influence sweeps across politics, religion, media, law, and even health. Think of this group as the top of each of the secret orders that are spread through the world, such as freemasonry. Very few members are ever invited to join the top of the ranks, and even less are aware that the 32nd Degree is not the highest one can progress in Freemasonry. You could presume that beyond the 32nd Degree it is no longer called Freemasonry at all. It's interesting to note that on the Great Seal (below) of the Federal Debt Note, the one dollar bill, you can count 33 Feathers on the left wing to the left, and 32 feathers on the wing to the right. Essentially, they are the "behind the scenes" playwrights who have molded the world into their stage. Either using their secret knowledge, or suppressing it from the public, they have controlled societies from the inside out, through infiltration, subversion, and control.
"It is not my intention to doubt that the doctrine of the Illuminati and the principles of Jacobinism had not spread in the United States. On the contrary, no one is more satisfied of this fact than I am." - George Washington, Freemason
It is said that these members consider themselves to be "enlightened" receiving their secret knowledge and understanding directly from a higher source, or due to a wider measure of human wisdom and history. Many theorize that higher source is "Lucifer" the Fallen Angel, who is the God in which they worship, believing the God of the Bible to be a "cruel and unjust God". When coupled with Spiritual Philosophy this translates to mean that they worship the Ego. Their history is rich with symbolism, as is the case with many secret orders. The all seeing eye has become their modern trademark, and it can be see in the one dollar bill, the opposite side of the eagle, atop a pyramid.
Below, in this Freemason image, you'll notice the "All Seeing Eye"
"Shouldst thou become a traitor or perjurer, let this sword remind thee of each and all the members in arms against thee. Do not hope to find safety; whithersoever thou mayest fly, shame and remorse as well as the vengeance of thine unknown brothers will torture and pursue thee."
Then in the Oath which follows he swears:
". . . Eternal silence, and faithfulness and everlasting obedience to all superiors and regulations of the Order. I also renounce my own personal views and opinions as well as all control of my powers and capacities. I promise also to consider the well-being of the Order as my own, and I am ready, as long as I am a member, to serve it with my goods, my honor, and my life . . . If I act against the rules and well-being of the Society, I will submit myself to the penalties to which my superiors may condemn me . . ." "In the name of the son crucified (i.e. the Pentagram, the illuminised man), swear to break the bonds which still bind you to your father, mother, brothers, sisters, wife, relatives, friends, mistresses, kings, chiefs, benefactors, and all persons to whomsoever you may have promised faith, obedience, and service. Name and curse the place where you were born, so that you may dwell in another sphere, to which you will attain only after having renounced this pestilential globe, vile refuse of the heavens! From this moment you are free from the so-called oath to country and laws: swear to reveal to the new chief, recognized by you, what you may have seen or done, intercepted, read or heard, learned or surmised, and also seek for and spy out what your eyes cannot discern. Honour and respect the Aqua Tofana (i.e. an imperceptibly slow poison) as a sure, prompt, and necessary means of purging the globe by death of those who seek to vilify the truth and seize it from our hands. Fly from Spain, Naples, and all accursed land; finally fly from the temptation to reveal what you may hear, for the thunder is no prompter that the knife, which awaits you in whatsoever place you may be. Live in the name of the Father, Son, and Holy Spirit. (The Trinity of Illuminism -- Cabalistic and Gnostic. The Father -- the generating fire; the Holy Spirit -- the Great Mother Nature, reproducing all things; the Son -- the manifestation, the vital fluid, the astral light of Illuminism)