Ralph uses his role as leader for the good of the group, but Jack uses it to gain and keep power for himself.
When Ralph is elected chief, he takes the role seriously. He wants to help the boys get rescued, and live comfortably while they are there. Building shelters and a signal fire are the most important things for him.
Jack sees it differently. When Ralph is elected leader, Jack jumps in immediately to focus on the use of power.
Jack was on his feet.
“We’ll have rules!” he cried excitedly. “Lots of rules! Then when anyone breaks ’em–” (ch 2)
Jack seems to only want rules for the fun of punishing anyone who breaks them. Ralph, on the other hand, just wants to get things done.
Ralph and Jack have conflicts over the difference in their emphasis. Jack and his hunters like to kill pigs, not just because they bring food but because killing them is fun. Ralph thinks that the fire is more important.
You hunters! You can laugh! But I tell you the smoke is more important than the pig, however often you kill one. Do all of you see?” (ch 5)
The difference in focus eventually leads Jack to break away from the group. When he is in charge, he is even more violent and impulsive. He beats boys for no apparent reason, just to prove that he has power. He and his tribe paint their faces and do pig dances, but there is no focus on practical things like keeping a signal fire going.
As leaders, Jack is tyrannical and Ralph is democratic. Neither one is effective though, because Jack rules by fear and Ralph can't keep control.