Happy Valentine's Day, everyone!
This is another cute holiday imported and instituted by candy companies and other industries in Japan. Even so, it's one day in the year that you can somewhat break societal expectations by giving a guy chocolates without too much embarrassment. In Japan, it's the women who give chocolate to their Valentines, and sometimes to close friends. Then, on White Day in March, the guys return the Valentine by giving girls white chocolate!
Of course, not all chocolate given on this day is "love-chocolate" (honmei-choko). Women also tend to give chocolate or small gifts to bosses, coworkers, or other people they are bound to by a sense of 'on-giri', so don't get too many ideas! This sense of indebtedness to coworkers or supporters means that at any opportunity, reciprocal gifts must be given, including on Valentine's Day. Make sure the person you are receiving from is really interested in you before making a move.
PROTIP: Men, you are expected to reciprocate on White Day. Write down what you got today from whom, so you don't forget to return the favour.
Also, don't panic too much. Or else, an XKCD comic will happen to you.