At the beginning of the story it was established that whoever had the conch would be the chief and could speak. Even though Piggy found the conch, Ralph became chief and blew the conch first to call all of the others who were on the island.
When Ralph and Piggy went to seek Jack and his tribe a conflict erupted between them which resulted in Piggy dying. The conch also broke and Jack proclaimed himself as chief.
The boys were out looking for the beast and when they thought they saw it they ran. This later caused a conflict between Ralph and Jack and they argued over who was a coward and who should be chief. After no one voted for Ralph to not be chief, Jack got mad and said he'd start his own tribe which would hunt whenever he would.
Jack and his tribe went to Ralph's people and their shelters to steal Piggy's glasses so that they could have a fire. Sam and Eric got beat up in the process and Piggy thought they were after the conch.
Simon had just come out of the forest and he found himself in the middle of Jack's tribes circle. They didn't know who it was and assumed it was the beast so they ended up killing him. They only realized it was Simon after the damage had been done.
At the end of the book Jack and his tribe set out to kill Ralph. The twins told Ralph that Jack had a spear sharpened at both ends. Then the island ends up in flames because of Jack and his tribes attempt to kill Ralph. At the end they get rescued .