The conch represents civilized society and order. The conch is used to organize the boys. When Ralph summons everyone to the first meeting, they decide on choosing a chief. Jack nominates himself. But some of the other boys recognize Ralph as a leader and this has something to do with the fact that he has the conch. "Let him be chief with the trumpet-thing." The conch also gives Ralph a sense of authority:
But there was a stillness about Ralph as he sat that marked him out: there was his size, and attractive appearance; and most obscurely, yet most powerfully, there was the conch. The being that had blown that, had sat waiting for them on the platform with the delicate thing balanced on his knees, was set apart. (Chapter 1)
Ralph stresses the necessity of keeping the fire going. He says that this is their best chance of being rescued. Another way of looking at this is that the fire is their best hope of reconnecting to "civilized" society. "Civilized" is in quotes because there is an atomic war going on. While talking about the fire, Ralph emphasizes the role of the conch in keeping order and civility:
We ought to have more rules. Where the conch is, that's a meeting. The same up here as down there. (Chapter 2)
In Chapter 5, a ship is spotted on the horizon. Piggy realizes the fire on the mountain has gone out. Ralph sprints to the top of the mountain. He is furious with Jack and the others for letting the fire go out. They have just returned from killing a pig, chanting "Kill the pig. Cut her throat. Spill her blood." Rather than staying to make sure the fire keeps going, they go hunting and return chanting like savage killers.
In Chapter 8, when Jack forms his own tribe and marches away, Ralph, Piggy, and some of the other boys look at the conch almost like a nostalgic symbol of the order that once existed on the island:
The group of boys looked at the white shell with affectionate respect. Piggy placed it in Ralph's hand and the littluns, seeing the familiar symbol, started to come back.
The conch is destroyed when Piggy is killed at the end of Chapter 11. Piggy is the intellectual on the island. The conch established order. This is symbolic on these two levels.
The fire is a means of connecting the boys to civilization. However, the fire almost destroys the whole island. As a primitive form of technology, the fire offers a rec-connection to civilization but also threatens destruction. Similarly, in the "civilized world," technology offers civilization but also proves destructive with the atomic bombs.
This all being said, Golding shows that people have the potential to be civilized and savage. He does not imply that we are "naturally" civilized. He complicates the matter by supposing if the boys are more or less civilized than their parents in the other parts of the world. The boys start out fine but devolve. With humanity killing each other with atomic bombs, what does that say about "civilized" society?