This is a great question. There is so much that could be stated that I will have to be brief.
Ralph represents civilization and order. After the plane crash, while most of the boys are exploring and having fun or just doing their own thing, Ralph is busy at creating order. He thinks about what can be done to maximize their chances of rescue as well as other ways to live on the island. In short, he is in the process of creating a society.
Jack is the opposite of Ralph. He represents chaos and the innate impulse towards lawlessness that is in all people. For example, after he lost the election of leadership to Ralph, he is upset at the loss of power. He is also obsessed with the hunt and he will do whatever he wants without consideration of the group. Base instincts consume him.
Simon is the most interesting character. He represent spirituality. He is the most religious out of the boys. He has a sense of right and wrong apart from society. He is also at peace with nature. He also utters perhaps the most profound words of the book "What I mean is . . . maybe it’s only us." When all the boys are fearful of the beast, Simon realizes that it might just be them. He intuits this. Finally, as a Christ-figure, he dies.