the island - The island itself is a symbol of the boys' isolation from the outside world; the community they form is a microcosm (a symbol on a small scale) of society in general.
the beach/the sea - The coastline where the less adventurous group stays at represents the need to cling to the familiar; as the boys look out to the horizon for a ship to eventually come along, the sea represents hope and rescue from their plight (return to civilization).
the conch - The conch also represents the respect for order and the need for a structured society. (Piggy's glasses represent the same thing.) Since the holder of the conch has the right to speak out, the conch could stand for a democratic government which upholds the right for individual personal expression.
the forest/ the underbrush/ the mountain - These places symbolize retreat from civilization into the unknown. Also, they are metaphors for the darker, more untamed side of the human psyche, as embodied by Jack and his followers.
the fire - Ironically, the fire symbolizes two opposing things. It represents warmth, nourishment and protection as the boys use it for cooking and making signals for rescue. On the other hand, when the forest catches fire, it also represents the destructive force of the mind when its power is not kept under control.
The characters, too, are archetypes as the boys polarize into two different groups representing the antipodes of civilization and savagery.