According to the Catholic Encyclopedia, there were at least three early Christian saints by that name. One was a priest in Rome, another a bishop in Terni, and the third, St. Valentine which almost nothing is known about, except that he met his end in Africa. Astonishingly, all the three Valentines were said to have been martyred on February 14.
Valentine who was a priest, served during the third century in Rome. When Emperor Claudius II decided that single men made better soldiers than those with wives and families, he outlawed marriage for young men. Valentine felt this was unfair so he celebrated marriages in secret. When the emperor found out he was thrown in jail and sentenced to death.