Korean written language, hangul, was created by King Sejong the Great and published in 1446. If you've been to the Gyeongbokgung, you've probably seen the golden statue of him on the middle of street at the front gate of the palace. Before this, Koreans, like many other East Asian nations, used Chinese character as their written form of language. Verbally however, Koreans have always spoken the Korean language, including the upper class.

Unlike Japanese though, Koreans did not modify the Chinese characters a whole lot, (though there were things called Idu script to assist the Korean verbal language be translated onto Chinese characters) and this was a great problem for Korean people to learn the written language. Add in the fact that grammar for Korean language is greatly differed from written Chinese, the literacy rate among the lower class people could not have been great.

It is recorded that King Sejong created Hangul in order for even the lower class of people can communicate through written means. However, officials did not use Hangul until much, much later as knowing the Chinese language was a status defining thing at the time and they were not willing to give that up. Hangul was treated as language for women and low class people for hundreds of years.

It isn't until the colonial era that hangul was "rediscovered". It was embraced with open arm during the Japanese rule as Hangul was something distinctively Korean and it was in fact much much easier to learn than Chinese characters.

Interesting thing here is that some Korean words are better served by being written in Chinese characters. Hangul itself is a manifestation of the verbal language (like alphabet) and does not inherently contain the meaning of a word the way Chinese characters do. This leads to some words being expressed in Chinese characters in Korean newspapers and sometimes even books to convey the word's meaning better. (A linguist can probably add/express a whole lot better than I can about this subject though)