What are the roles of rosencrantz and guildenstern in the play, HAMLET?

Apart from what Lili has already said, if you have time, it might be a good idea for you to read Tom Stoppard's Rosencrantz and Guildenstern are dead. Here is the e-text: http://books.google.es/books?id=_26yV2gOAYsC&dq=rosencrantz+and+guildenstern+are+dead&printsec=frontcover&source=bn&hl=es&ei=I030SviMGIrG4QbM0PXdAw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CCkQ6AEwBQ#v=onepage&q=&f=false