The boys were on a plane, and then crashed. No adults survived, and it was only young children, which they called littluns, and the older boys. The only gender on the island was male. When the boys have meetings, they use a conch to indicate who can talk. The conch represents power.
Jack, who is the leader of the choir boys, takes care of the fire, which the fire is being used as an SOS Call. But the boys accidentally let the fire get out of control. So the whole island accidentally caught on fire. One of the littluns got burned alive, the one that had a birthmark on his face.
When Jack and his Hunters kill a pig, and they eave the skull of the pig for the “beast”, who is not real. This is the start of the beginning of the end of Jack and his Hunters decent into savagery.
Simon is the first to finally figure out that there is no beast, but the figure that they are talking about is the dead pilot, that took a parachute out of the plane. So Simon runs out of the woods to go tell the rest of the boys. But the group thinks he is the beast, and they go and kill Simon, without realizing it is him.
Piggy goes to Jacks tribe to ask for his glasses back, because they stole them. And Rodger pushes a big rock boulder knocking Piggy into the water, which Piggy ends up dying.
At the end of the book, the rest of the boys who survived on the island, ended up getting rescued by a naval officer. The naval officer rescued them on a boat. He noticed the fire that was on the island, and he went to the island and saved all the surviving boys.