GUIDO CAVALCANTI (c. 1250-1300) was a Florentine exiled from that city to Sarzana for his pro-papal politics. He died in Florence shortly after returning from exile.
Dante dedicated his Vita Nuova to Cavalcanti, and the two are considered the greatest poets of the Florentine school. He was one of the first to create a new style of poetry, the dolce stil nuovo, that was to inspire Dante.
For a brief biography see Decameron Web.
For further information see Wikipedia.