Korea's traditional clothing is known as hanbok (한복). Men wear a traditional jacket and pants known as jeogori (저고리), the women wear a long skirt and woman's jeogori.
Over time, especially since the Korean War and the industrialization of the sixties and seventies, hanbok style clothing became less common and became replaced with Western fashions, known as yangbok (양복).
Nevertheless, you will occassionally see Koreans walking down the street or through subways, while wearing hanboks. Koreans still wear their traditional clothing on special occassions, most notably, weddings, sometimes on Lunar New Year, Chuseok, the birthdays of one's parents, someone's 70th birthday, and other signficant events in Korean culture.