Until Chaucer wrote poetry about "Valentines" in the 14th century, this holiday was for the martyred Christian saint(s) Valentinus. It was first established by Pope Gelasius in 496AD and later deleted from the general Roman calendar of saints in 1969 by Pope Paul VI.
It is celebrated in countries around the world, but mostly in the West.
As noted above, the day first became associated with romatic love in the circle of Geoffrey Chaucer in the High Middle Ages, when the tradition of courtly love flourished. By the 15th century it had evolved into an occaision in which lovers expressed their love for each other by presenting flowers, offering confectionary and sending hand-written greeting cards (known as "valentines").
Modern Valentines Day symbols include the heart shape outline, doves and the figure of the winged Cupid. Since the 19th century, hand-written valentines have given way to mass-produced greeting cards.