He's one of the most famous philosophers in recent American history. He taught at Harvard, wrote numerous works, and was famous for his political thought.
Perhaps his greatest contribution was his theory of a just society. In order to determine what you believe is a just society, you need to put yourself in the "original position." This involves a thought experiment in which you place yourself behind a "veil of ignorance." Rawls describes this experiment as one in which "no one knows his place in society, his class position or social status, nor does anyone know his fortune in the distribution of natural assets and abilities, his intelligence, strength, and the like. I shall even assume that the parties do not know their conceptions of the good or their special psychological propensities. The principles of justice are chosen behind a veil of ignorance." [A Theory of Justice]
So what he's basically saying is that if you imagine that you don't know your gender, age, race, country of origin, social/economic standing, personality type, or anything else about yourself, then and only then can you think about what type of rules a just society should have. Rawls says that this would lead to people accepting the view that we should maximize the welfare of society's worst off, because you might end up being one of them. The point is, you don't know, so you'd probably establish rules that kept everything as fair as possible. This wouldn't necessarily lead to socialism; you might gamble a bit and have some people be richer than others on the chance that you could end up one of them. What it would lead to is a set of rules that ensures that nobody is exploited or treated unfairly.
He died about five years ago. I would have loved to have attended a conference or a lecture and heard him speak; a few of my professors were lucky enough to have had that privilege. He was an enormous contribution to social philosophy.