It was introduced by King Sejong in the 15th century in an effort to increase literacy. Sejong boasted that a clever person could learn Hangul in a day, while a fool could learn it in a week. However, Hangul wasn't widely used until the early 20th century.
In South Korea, Hangul is used alongside Chinese characters, or Hanja in Korean. However, in North Korea, only Hangul is used to write Korean as Chinese characters are not allowed in official publications, because they are seen as "bourgeois".