Ralph and Jack differ over the idea of power, mainly. While Ralph is constantly striving to do the right thing w/ his leadership position, Jack exploits his power, using it towards less noble means--i.e. debauchery, murder, manipulation--in a word, sheer corruption. But the two really butt heads b/c, in a way, they are also remarkably alike. Check the link--it shows this character symmetry/conflict really well.
During the confrontation at castle rock Ralph said, and I quote... "You came sneaking up like a thief and stole Piggy's glasses." Jack replied "Say that again!" Ralph obliged and repeated "Thief! Thief!" Jack's reaction was to rush forward and stab at Ralph's chest with his spear.
Ralph was fighting with Jack in the dark. The confrontation was sparked by their ongoing power struggle and disagreements over leadership.
During the meeting when Jack talks of hunting the beast Ralph describes Jack's hunters as, "boys armed with sticks." Later, during the confrontation at Castle Rock, Ralph calls Jack a Thief.
They both don't agree with each other and they don't get along because Jack doesn't care about anyone but himself.
In Chapter 10 of "Lord of the Flies," Ralph and Eric were hunting down Jack's tribe to retrieve Piggy's glasses. They were not actually hitting anyone, but there was a physical confrontation between them and Jack's tribe on the beach.
During the confrontation in "Lord of the Flies," Samneric betray Ralph's trust and align themselves with Jack's tribe. They give away Ralph's hiding place to the other boys, leading to Ralph being hunted like prey. Ultimately, Samneric are forced to participate in the hunt for Ralph against their will.
During the confrontation on the causeway leading to Castle Rock Ralph first called Jack a "Thief" and later when his temper was pushed beyond breaking point he screamed at Jack, "You're a beast, and a swine and a bloody, bloody thief!""
The conflict between Ralph and Jack is directly an example of Reason Vs. Savagery or Superego (knowledgeable) Ralph Vs. Id (apart of your mind that is selfish and wants everything now) Jack.
When Ralph recognised from their silhouettes that Samneric were on guard duty on top of castle rock he took the risk of climbing it to speak to them. Ralph knew that Samneric had remained loyal right up to the point when they had been catured and forced to comply with Jack's wishes.
Most of the boys have joined Jack's tribe leaving only Piggy and the twins, Samneric, remaining loyal to Ralph.
Ralph and Jack never liked each other. Once they met and Jack wanted to take the power from Ralph there was instant friction between the two. It's was obviously Ralph's job to be chief and Jack just expected to get it instead of Ralph.
Jack thinks ralph has no leadership skills, and essentially he thinks he is better because he can sing and hunt and ralph cannot