Some may suggests that the Illuminati is more a team of heroes instead of an organization, but considering how they operated I think the later makes more since. Regardless, they were a group of heroes that consisted of Dr. Reed Richards, Iron Man, Professor X, Dr. Strange, Black Bolt and Namor. They were a secret organization that operated in the shadows, sharing info and strategy. Each member was a representative of aspect of the superhero community.

Namor is the king the underwater civilization Atlantis, Dr. Strange represented the mystics and “outsider heroes,” Dr, Richards being one of the smartest super-humans, Professor X was the leader of the X-Men, world class telepath and a well known mutant. Black Bolt is king of the inhumans and Iron Man is like Dr. Richards one of the smartest men, as well as being the leader of the Avengers. Black Panther is king of an important nation, Wakanda was offered membership but initially declined. He was against the creation of the Illuminati and predicted that ” the people of the Earth had feared super-humans deciding to take the Earth’s affairs solely in their own hands.”

Iron Man was the mastermind behind the groups formation. He believed that could combine their considerable resource to defend Earth. He felt that “since alien races viewed the Earth’s heroes as its law enforcement, they should start acting like it.” The Illuminati would go on to work with the government (mainly SHIELD) to banish the Hulk to another planet and to register super-humans during the Civil War arc. Black Panther’s prediction was accurate as the Illuminati’s decision to shoot the Hulk into space came back to haunt them when he returned for revenge. The Hulk defeated Black Bolt, the X-Men, Iron Man, the Fantastic Four and Dr. Strange.

While the Black Panther was right and the Illuminati did overreach causing unnecessary problems, they had good intentions. However, like many of the major organizations in Marvel they were shady. Ultimately they didn’t function properly and the original members are no longer part of the group.

Source: Marvel wiki