This is a reference provided for those reading a translation that uses honorifics. (They are not part of the name of the characters, and should not be treated as such.) Information courtesy of Easy Going Scans.
Hope this helps!
Nim - A formal way of addressing someone typically older and/or more respected than you.
Ssi - A formal way of addressing someone you do not know or are unfamiliar with.
Nuna - A friendly way of addressing an older woman near your age if you're a guy.
Hyung - A friendly way of addressing an older man your age if you're a guy.
Unni - A friendly way of addressing an older woman near your age if you're a girl.
Oppa - A friendly way of addressing an older man near your age if you're a girl. Oppa is also used (though not synomymously) by girls addressing their boyfriends.
Ajumma - Older woman typically aged between mid/late 30's ~early 50's. Calling someone younger than mid 30's this is rather offensive.
Ajussi - Older man typically aged between mid/late 30's ~ early 50's. Korean men, especially in their 30's, prefer to be called hyyg/oppa by those younger than them rather than ajusshi.
Sunbae - Is used to address senior colleagues or mentor figures, e.g. students referring to or addressing more senior students in school.
Hubae - Is used to refer to juniors. However, the term is not normally addressed to them directly, and is mainly used in the third person.
This is nearly identical in Japanese with Senpai/Kohai honorifics.
Naeuri - Was used by commoners in the Joseon Dynasty to refer to people of higher status but below "His Excellency".
Seonsaeng - Commonly translated "teacher", has much more formality, and is used to show respect to the addressee. It is related to the Japanese honorific, "sensei" and shares the same pair of Chinese characters, representing 'mister' or 'one born earlier' (literal).