From the wikipedia article on the day:

St. Valentine’s Day falls on February 14, and is the traditional day on which lovers in certain cultures let each other know about their love, commonly by sending Valentine’s cards, which are often anonymous. The history of Valentine’s day can be traced back to a Catholic Church feast day, in honor of Saint Valentine. The day’s associations with romantic love arrived after the High Middle Ages, during which the concept of romantic love was formulated.

The day is now most closely associated with the mutual exchange of love notes in the form of “valentines.” Modern Valentine symbols include the heart-shaped outline and the figure of the winged Cupid. Since the 19th century, the practice of hand writing notes has largely given way to the exchange of mass-produced greeting cards. The Greeting Card Association estimates that, world-wide, approximately one billion valentine cards are sent each year, making the day the second largest card-sending holiday of the year behind Christmas. The association also estimates that women purchase approximately 85 percent of all valentines.