We'll never really know exactly who he was or what he did, but the stories surrounding this popular saint have contributed to the Valentine's holiday traditions. The Catholic Church recognizes at least three different saints named Valentine or Valentinus. Although there are two main stories about St. Valentine, it is possible they evolved from the life of one man.
More Than One Saint Valentine?
Some believe that he was a Catholic priest and physician in Rome who was beaten to death under the Christian persecution of Emperor Claudius II, who ruled from 268 to 270 A.D. Some accounts say that before he was martyred, he was thrown in jail where he befriended the jailer's blind daughter and restored her sight. The day of his execution, he left a note for this girl to thank her for her friendship and signed it "From your Valentine." Some accounts say that he rubbed ointment in her eyes on several occasions or prayed for her incessantly to heal her. The other legend is that he left a blossom in his farewell note, and that blossom healed her. This note may have been the first valentine.
The second, and perhaps more well-known, story is that St. Valentine was a priest who married couples during the reign of Claudius II, who outlawed marriage in order to keep his soldiers focused on war rather than wives. This priest was eventually caught and martyred. Some say that while he was in jail, lovers slipped notes and flowers into his cell to thank him for believing in marriage and love.
Matyrdom of Saint Valentine
Both St. Valentines are believed to have been martyred on Feb. 14. It's interesting to note that, in ancient Rome, Feb. 14 was the day honoring Juno, the queen of the Roman gods and goddesses, who was also the goddess of women and marriage. Feb. 15 was the start of the Feast of Lupercalia, during which Roman boys drew the names of Roman girls from a jar in order to choose a partner for the festival. In the early Christian church, the Roman priests tried to Christianize the pagan holiday by placing the names of saints in the jar so that those choosing would draw a patron saint for the year instead of a maiden's name. Pope Gelasius declared Feb. 14 the day to honor St. Valentine in 496 A.D.
Things You Didn't Know About St. Valentine
Valentine Traditions
During the Middle Ages, popular thought was that birds paired midway through February, thus Feb. 14 lent itself to being a day for lovers. In England and France, St. Valentine was one of the most popular saints. Many traditions developed, such as children dressing up as adults and going from home to home singing verses like:
Good morning to you, valentine;
Curl your locks as I do mine ---
Two before and three behind.
Good morning to you, valentine.
Flowers, poems and small gifts have been a Valentine's Day tradition for centuries. The oldest known Valentine was given by the Duke of Orleans to his wife after he had been taken prisoner by the English in 1415.
In the United States, Esther Howland is credited with sending the first valentines. Commercial valentines were first produced in the 1800s.
Sources
These were the most informative sites I found about St. Valentine and the holiday traditions:
History Channel.Com
Catholic Encyclopedia
History, Story and Traditions