Women give chocolates (either homemade or store bought) to their loved ones. A "true-love" chocolate is called Honmei-choko.
Chocolate is also given to their bosses, co-workers, or male friends (whom they have no romantic interest in) just as a sign of friendship or gratitude. This is called Giri-choko which means chocolate given because of obligations. The concept of "giri" is very Japanese. It's a mutual obligation that the Japanese follow when dealing with other people. If someone does you a favour, you feel obligated to do something for that person in return.
A new custom on Valentine's Day is called Gyaku-choko (reverse chocolate). Men give women chocolates on Valentine's Day. Also, Tomo-choko (friendship chocolate) is popular among Japanese girls. They give their friends chocolate on Valentine's Day.
Men who received chocolates/gifts on Valentine's Day are supposed to return gifts to women on White Day (March 14), a day unique to Japan. Gifts for White Day are usually sweets, i.e. candies or cookies.
Screenshots are from the anime series Super GALS!
Screenshots are from the anime series Fruits Basket