Who are Rosencrants and Guildenstern? Why have then been invited to the castle? What motives do they have for carrying out the king's request?

Rosencrantz and Guildenstern are Hamlet's childhood friends whom is uncle invites to the castle to spy on Hamlet. Because of their history, King Claudius is hoping that they will be able to gain Hamlet's confidence. However, Hamlet see through their ruse and they become people for Hamlet to mock. Their motives are purely materialistic. They receive a monetary stipend for their deception and they are also hoping to further their status with King Claudius.