Hanja (한자/漢字) is basically the Korean word for Chinese character (Hànzì). (the Character Han (漢) refers to the Han people which is the main and largest ethnic group in China.)In specific, its those Chinese character that is used in Korea as the dominate writing system even after the invention of the Hangeul (한글) alphabet by King Sejong in the Choseon dynasty.
In modern times, Hanja has been replaced by Hangeul. There are some very valid reason for this:
1. Hanja often represented borrowed words from Chinese and thus does not address the native Korean words. This is a hindrance when trying to understand the Korean language which is polysyllabic and not tonal unlike Chinese which is monosyllabic and tonal.
2. Hangeul is much easier to read . . .(at least to those who knows how to read it.) The adoption of Hangeul
But for all these reason, Hanja still is a valuable part in Korean culture and thus should not disappear from Korea all together. Many ancient art and literature requires some understanding of Hanja to appreciate. It has been a integrated part of its government system and religious structure. Sure it's foreign, but so are cars yet those are kept. It provides diversity in language. And from an academic standpoint it shows diligence in study. Plus it prevents confusion. Take the Vietnamese Quốc Ngữ. While it allows readers to understand the sounds of the words, but it creates confusion because a single syllable can have different meaning by changing the slightest accent. For learners this can mean difficulty in remembering certain words. For Korean, its worst! Korean is not a tonal language and this there is no way to discern a syllable that has different meanings. Trying to change Chinese based words would be to cumbersome as it is already part of the society. It would be like the convoluted practice of replacing Latin base words with native English words or Japanese On-yomi (音読み) with western equivalents to fit in. The solution is to allow some hanja (certainly not all!) to remain part of the Korean language much like how Japanese Kanji is written with Hirigana (平仮名) and Katakana (片仮名). Hanja is an undeniable part of Korean Culture.