Hanja is the Korean word for Chinese characters. Korean script is now mainly written using Korea's native phonetic/alphabetic script - Hangul (한굴) but originally used only Hanja. Hangul has gradually replaced the majority of Hanja though in some contexts, including Korean names, they are still used extensively today. For instance, South Korea uses both hanja and hangul in its newspapers. North Korea uses hangul only.
These are the same characters known as hanzi in Chinese and kanji in Japanese (and chu nom in Vietnamese). The style of the hanja characters is much closer to that of hanzi than kanji.
The word Hanja is represented by two Chinese characters, 漢字. The first is "han", which means China. The second is "ja", which means letter or character. So together Hanja means "Chinese characters".
Sources: My memory, suggestions from Shro0m, stealing bits of ojnk's
Characters provided by another noder whose name slips my mind. Apologies!