Korean Honorifics

Korean, like Japanese, has an extensive system of honorifics, words usually appended to the ends of names or pronouns to indicate the relative ages and social positions of the speakers. Immigrants to the Koreas often find this idea difficult to grasp, but it is a very important feature of language. Using the wrong honorific can and will cause offense.

Generic honorifics

Si (씨; pronounced shee): When appended to a full name or personal name, it indicates that the speaker considers the speakee to be of the same or a lower social level. When appended to a surname, it indicates that the speaker considers themselves to be of a higher station than the speakee, and has a "distant" connotation that is considered rude if applied to elders.
Gun (군): Used in the same context as Si but applied to unmarried men/male minors only.
Yang (양): Used in the same context as Si but applied to unmarried women/female minors only.
Seonsang (선생): Very respectful, commonly translated as master or teacher. On its own, it is applied to doctors and teachers. Shares the same Chinese characters as the Japanese word sensei.
Gwiha (귀하): Reserved for letters and messages, when referring to the recipient. Dropped for informal situations.
Gakha (각하): Reserved for high-ranking politicians, including the president.
Seonbae (선배): Used in a company for senior employees, or in schools for those in higher classes. May be used as both an honorific and a title. Equivalent to Japanese -senpai.
Hubaei (후배): Junior; may be used as an honorific or a title. Equivalent to Japanese -kohai.
Junha (전하): Archaic honorific from the Choson dynasty, used to refer to a King. Usually translated His Excellency.
Nari (나리 or 나으리): Archaic honorific from the Choson dynasty. Used by commoners to refer to nobles below the king.
Nim (님): Reserved for anyone of a higher station than the speaker, or those whom the speaker holds in high regard. For example, students may call their upperclassmen Seonbae-nim. It may be dropped if the parties involved are close enough that such formalities are unnecessary, such as with family and close friends. However, this honorific is mandatory for the formal use of the word Seonsang--Seonsang-nim is respectful, but just Seonsang is considered (in some cases) very rude.
Dongmu (동무; "Tongmu" according to the McCune-Reischauer transliteration used in North Korea): The Korean equivalent to the word "comrade". In North Korea, just like in the Soviet Union and many other Eastern Bloc countries, this word replaced most of the existing titles and honorifics as a standard form of address meaning "fellow revolutionary", whereas in the South it has mostly gone out of use due to its association with the Dirty Communists.

Pronouns

Oppa (오빠): For a female's older brother (literally and figuratively) and for older men (but old by too much) whom the women trust. Women often use it for their boyfriends as well. The female equivalent is "Unni". Men never use either of these.
Hyung (형): For a male's old brother (literally and figuratively) and for men they're close to/respect. The female equivalent is "Noona". Women never use either of these.