Three different St. Valentines were mentioned in early lists of Christian martyrs, but not much is really known about any of them. Eventually, their legends got conflated, and all three came to share the same feast day, February 14. The St. Valentine that we know the most about was a third century priest and physician in Rome. He, along with his family and a few friends, comforted the martyrs during the persecution of Emperor Claudius II, the Goth. Eventually, Valentine was also arrested, condemned to death for his faith, beaten with clubs, and finally beheaded. That was said to have happened on Feb. 14, AD 270. He was buried on the Via Flaminia, and, later, Pope Julius I (333-356) built a basilica at the site which preserved St. Valentine's tomb. Archeological digs in the 1500s and 1800s found evidence of the tomb of St. Valentine, but his bones had long since (in the thirteenth century) been transferred to the Zeno chapel in the church of Saint Praxedes (Santa Prassede), where they remain today. (Santa Prassede is near the Basilica of St. Mary Major on Via S. Martino ai Monti.) Another small church was built near the Flaminian Gate of Rome, now known as the Porta del Popolo. In the 12th century the gate was called "the Gate of St. Valentine," according to an early British historian William Somerset (William of Malmesbury, died 1143).
The second St. Valentine was the Bishop of Interamna (now Terni) located about 60 miles from Rome. On the orders of a prefect named Placidus, he too was arrested, scourged, and decapitated, also during the time of Emperor Claudius II.
The third St. Valentine suffered martyrdom in Africa with several companions. Nothing further at all is known about this saint.
The connections of the martyrs with love and romance is very tenuous. There are legends, but no solid evidence, that the first Valentine mentioned above converted the daughter of his jailer, cured her blindness, and and wrote her a final note signed "Your Valentine" and that he -- or maybe the second one -- performed marriages for soldiers after Claudius II had forbidden them. Some authorities note that the feast of the three St. Valentines almost coincided with the Roman pagan Lupercalia festival (February 15) and with the festival of Juno Februata, during which single men and women drew lots and could be paired for pleasure for the succeeding year. They suggest that the pagan pairing custom was incorporated into Christian legend. In fact, the early church did hold the same kind of ceremony, but instead of pulling the name of a potential partner out of the jar, Christian teenagers had to draw lots bearing the names of saints that they were supposed to emulate for a year.
Today's more popular Valentine's Day practices actually seem to have originated in England and France of the medieval period. It was thought that on February 14 birds began to pair and that the day was therefore an appropriate occasion for smitten youths to exchange love letters and send small romantic gifts: French and English literatures of the fourteenth and fifteenth centuries abound with stories of the practice. Those who sent such tokens appear to have called each other their "Valentines". The most famous exchange of the kind is preserved in the "Paston Letters". Dame Elizabeth Brews, trying to arrange a match for her daughter Margery, wrote to John Paston, the favored suitor: "And, cousin mine, upon Monday is Saint Valentine's Day and every bird chooses himself a mate, and if it like you to come on Thursday night, and make provision that you may abide till then, I trust to God that ye shall speak to my husband, and I shall pray that we may bring the matter to a conclusion." A short time later Margery herself wrote to Paston addressing her letter: "Unto my rightwell beloved Valentine, John Paston Esquire".
By the early 19th century, the practice of sending "valentines" had died out except in some parts of England, but in the late 1840s, Esther A. Howland, the 19-year-old daughter of a Worcester Massachusetts stationer and bookseller received a valentine card from one of her father's business associates in England. She saw a business opportunity, and within a year her brother William had accumulated $5000 worth of orders for her elaborate hand-made cards. Business doubled the next year, and by the early 1870s they had an assembly line and their annual revenues were over $100,000. Prices for hand-made Howland cards ranged from $5 to $35 -- a great deal of money in those days. But the success of their valentine trade spawned many competitors, some of whom sold much cheaper machine-made cards. The Howlands eventually had to sell out to one of the mass producers. The US market continued to grow, and now the US Greeting Card Association projects sales of over 900 million valentine cards per year in the US. Hallmark Cards, Inc., founded by Mr. Joyce C. Hall in Kansas City, Missouri in 1910, leads that market. Candy and flower sales are also booming. And now, of course, you can even buy or send valentines over the Internet, simple free animated cards or very elaborate or expensive customized stuff you can order up for hand delivery.
St. Valentine in the Catholic Encyclopedia: http://www.newadvent.org/cathen/15254a.htm
The short account of St. Valentine from Voragine's notoriously fanciful "Golden Legend" (written in 1275 AD): http://www.fordham.edu/halsall/basis/goldenlegend/GoldenLegend-Volume3.htm#Valentine
The pagan connection: http://www.religioustolerance.org/valentine.htm
"Annie's Valentines History Page" contains a wide range of Valentine lore, including the definition of a "penny dreadful": http://www.annieshomepage.com/valhistory.html
History of Valentine Cards: http://antiques.about.com/hobbies/antiques/library/weekly/aa020700.htm
One of many sites offering Valentine crafts, projects, recipes, artwork: http://www.teelfamily.com/activities/valentine/Default.htm
Order vintage Valentine postcards: http://www.xmission.com/~tssphoto/val_info.html
Recipe for the Ultimate Valentine's Day Chocolate Truffle: http://holidayrecipe.com/az/UltimateValentinesDayChoco.asp
The Loveland, Colorado, Valentine remailing program: http://www.loveland.org/valentine/
I don't know how real this is, but Oprah recommended it last year: Apply for citizenship for yourself and your beloved in the "UNO recognized" Independent Principality of Saint Valentine, supposedly on the Swiss-Austrian border. $55 gets you an e-Certificate and two passports (express service $18.20 extra). http://stvalentine.com/index1.htm
P.S.: Men should not forget that in Italy we also celebrate Festa della Donna (International Women's Day), on March 8. On that day we give blooming Mimosa to our wives, lovers, mothers, daughters, and female friends as a sign of our personal respect for them and to demonstrate our solidarity with their efforts to help oppressed women worldwide.