Valentine's day historically came from a Roman holiday called Lupercalia. This holiday was pretty far removed from what we celebrate now. This holiday was often associated with drinking festivities for monks and the random pairing of men and women by lottery. After around the 14th century, the celebration of Lupercalia was banned by the catholic church, and Valentine's day took its place over the next few centuries.
In America, valentines day is a general couples holiday. Both men and women, boys and girls, that are sweethearts, will exchange gifts and chocolates with one another in celebration of love. This romantic idea of the holiday is shared by most people.
In Japan, it seems that the holiday differs a little bit. Instead of it being a holiday where both the boys and girls exchange chocolate and gifts, it is just the girl confessing her affection, and giving gifts. Then later on White day, the boy gives gifts and chocolates to the girl.