King was a writer, speaker and political activist who promoted non-violent resistance to unnacceptable rules of law and conventions of society.
He helped to dignify a social movement that was under attack from the start by refusing to respond to violence with violence, to hate with hate. He once said that darkness cannot drive out darkness, only light can do that. Hate cannot drive out hate, only love can do that.
He spoke well and by most accounts did very well at living up to his word.
He was jailed for protesting and he was honored by JFK and Lyndon Johnson. He was attacked and shot, and he was praised for his eloquence, his compassion, his integrity, his courage, his will, his faith and his intelligence.
He graduated early from high school and recieved a BA before he was 20, I believe, and an MA and a Phd. (in separate programs) by the time he was 27.
He had a family. And he dedicated himself, despite the costs, to a life of social change and service.
I think the most lasting effect King had on America, and the world, was that he used powerful words to get his message across. He was a phenomenal public speaker, one of the best we've ever had, and his words are still inspirational today. We are lucky we have recordings of his speeches.
King is best known for the kinds of things mentioned above. However, we must note that he was also more critical of the US government than we tend to remember. He was extremely opposed, for example, to the Vietnam War. This sort of stand, along with his push for economic equality, would surely have led to a major decline in his popularity among most white Americans had he not been killed.
King was able to use his charisma and leadership skills to bring the issue of civil rights to the forefront of the national agenda. He also helped to inculcate the movement with a spirit of non-violent direct action. Later in his life, he began to move to issues of economic equality before his assassination.
Dr. Martin Luther King Jr. was the leading civil rights activist of the 1950s and 1960s--a proponent for equal rights for African Americans and for peaceful and non-violent protests much in the same vein as the Indian activist, Mahatma Gandhi. King's assassination in 1968 made him a martyr for the civil rights movement and a hero to black and white men alike.