Who are Rosencrantz and Guildenstern?

The characters Rosencrantz and Guildenstern are old friends of Hamlet from his childhood. After witnessing Hamlet's strange behavior, King Claudius summons the two of them to find out the cause of his distressing "madness."
We first meet Rosencrantz and Guildenstern in act II, scene 2 of the play. They are welcomed immediately by their old friend, but it is not long before Hamlet sees through their fawning language and recognizes them to be spies of his uncle. It is here that Hamlet realizes he can trust nobody (except maybe Horatio).
In all the following encounters between Hamlet and Rosencrantz and Guildenstern, the prince is cold and distant from them. He tells his mother that he trusts them as much as he trusts a poisonous snake. Later on, King Claudius sends them, along with Hamlet, to England with a letter instructing the English King to execute the prince. That they probably did not know the contents of the letter does not matter, and Hamlet changes the message to instruct the king of England to kill his former friends. It is clear that Hamlet's paranoia has taken control of him, and he is willing to coldly sacrifice two men who were once his close friends. When their ship is attacked by pirates, Hamlet leaves the two to die and returns to Denmark. We learn from the English ambassador, at the end of the play, that Rosencrantz and Guildenstern have been killed.
Hamlet's treatment of his former friends highlights his erratic and uncertain decision-making process. While he was quick to send Rosencrantz and Guildenstern to their deaths, he hesitates and cannot seem to make up his mind over what to do about Claudius, who is much more guilty than the other two.

Rosencrantz and Guildernstern are characters in Shakespeare's play Hamlet.
Rosencrantz and Guildernstern are childhood friends of Hamlet's. They are warmly welcomed by Hamlet in act 2, scene 2 at a time when the title character seems most in need of emotional support.
However, Hamlet quickly realizes that both Rosencrantz and Guildernstern have been summoned to try to find out why Hamlet has been acting weird. The ulterior motive to Rosencrantz and Guildernstern's appearance makes Hamlet instantly begin to distrust them. Additionally, because Hamlet's strange behavior stems from the information he learns when he sees the ghost of his dead father (that Claudius killed Hamlet's father so he could take the throne and marry Queen Gertrude) and because Hamlet has vowed to enact revenge by killing Claudius, Hamlet decides that he needs to "deal justly" with Rosencrantz and Guildernstern.
Eventually, Hamlet manages to trick them in an act of deception that leads to their deaths.