I have only heard people in Japan refer to "Valentine's day" but my Apple dictionary gives three meanings:
All of which are compatible with my understanding of the day:
St. Valentine's Day is a feast day in the Anglican church (among others). It began as a celebration of an early Christian saint named Valentinus. He was imprisoned for performing weddings for soldiers who were forbidden to marry. He is said to have healed the daughter of his jailer before his execution and wrote, "from your Valentine" as a farewell to her. The day's association with romantic love grew in the circle of Geoffrey Chaucer in the Middle Ages. By the 15th century, it had evolved into an occasion in which lovers expressed their love for each other by presenting flowers, offering confectionery, and sending greeting cards (known as "valentines"). Since the 19th century, handwritten valentines have given way to mass-produced greeting cards. (IOW: Where as it was highjacked by greetings card companies in Europe, this was done by the chocolate companies in Japan.)
However the gifts are not necessarily "the valentines" for everybody: The expression "Would you be my valentine?" (or lover) echoes the legend and its use in Shakespeare's Ophelia ("To-morrow is Saint Valentine's day,All in the morning betime, And I a maid at your window, To be your Valentine.")
Incidentally, I am not sure about other Western countries but in the UK it is the day when you can declare your love anonymously to the person you admire: The cards should be unsigned, with possibly a clue to add to the fun and the newspapers enjoy the extra revenue from publishing hundreds of anonymous personal messages.
(This is largely borrowed from a longer entry in Wikipedia)