St. Valentine is the Catholic Patron Saint of Lovers, a former Catholic Bishop.  There are several different versions of his story and how he can be the patron saint of lovers.  One version of the story is that he secretly married soldiers and their sweethearts despite the fact that it was illegal for a Roman soldier to marry.  Eventually he was caught and thrown into prison and executed.

The idea of exchanging cards on Valentine's Day, is said to have started because Valentine's loved ones would pass notes/cards expressing their love for him through the prison bars.

But this is probably not the real reason.  The real reason is probably related to the combination of the religious holiday and the pagan Roman rituals of Lupercalia...

In ancient Rome, February 14th was a holiday to honor Juno. Juno was the Queen of the Roman Gods and Goddesses. The Romans also knew her as the Goddess of women and marriage. The following day, February 15th, began the Feast of Lupercalia.

The lives of young boys and girls were strictly separate. However, one of the customs of the young people was name drawing. On the eve of the festival of Lupercalia the names of Roman girls were written on slips of paper and placed into jars. Each young man would draw a girl's name from the jar and would then be partners for the duration of the festival with the girl whom he chose. Sometimes the pairing of the children lasted an entire year, and often, they would fall in love and would later marry.

(Source: http://www.pictureframes.co.uk/pages/saint_valentine.htm)

American Culture (Traditions) on Valentine's Day:

1.  In Elementary School, children exchange cards with all members in their class.

2.  In Middle and High School, and beyond, only lovers exchange cards and/or gifts.  It is traditional for a man to give chocolate and a rose to his sweetheart.