Korean romanization is a system for representing the Korean language using the Latin script. Korea's alphabetic script is called Hangul, and is occasionally written using the combination of, or indepentently from Hanja.
Romaja literally means Roman letters in Korean, and refers to the Latin script. "Romaja" is not to be confused with "romanization". The former can be applied to any use of the Latin script in Korean text—whether for Korean or non-Korean words or names—while the latter refers to writing Korean words using the Latin script: either romanizing individual words in a Korean text, or writing an entire Korean text in the Latin script.
Many romanization schemes are in common use:
- McCune-Reischauer (MR; 1937?), the first transcription to gain some acceptance. A slightly modified version of MR was the official system for Korean in South Korea from 1984 to 2000, and yet a different modification is still the official system in North Korea. MR uses breves, apostrophes and diereses, the latter two indicating orthographic syllable boundaries in cases that would otherwise be ambiguous.
- Several variants of MR, often also called "McCune's and Reischauer's", differ from the original mostly in whether word endings are separated from the stem by a space, by a hyphen or not at all; and if a hyphen or space is used, whether sound change is reflected in a stem's last and an ending's first consonant letter (e.g. pur-i vs. pul-i). Although mostly irrelevant when transcribing uninflected words, these variants are so widespread that any mention of "McCune-Reischauer romanization" may not necessarily refer to the original system as published in the 1930s. MR-based romanizations have been common in popular literature until 2000.
- The ALA-LC / U.S. Library of Congress system is based on but deviates from MR. Unlike in MR, it addresses word division in seven pages of detail. Syllables of given names are always separated with a hyphen, which is expressly never done by MR. Sound changes are ignored more often than in MR. ALA-LC also distinguishes between ‘ and ’.
- Yale (1942): This system has become the established standard romanization for Korean among linguists. Vowel length in old or dialectal pronunciation is indicated by a macron. In cases that would otherwise be ambiguous, orthographic syllable boundaries are indicated with a period. Indicates disappearance of consonants.
- Revised Romanization of Korean (RR, also called South Korean or Ministry of Culture (MC) 2000): Includes rules both for transcription and for transliteration. South Korea now officially uses this system which was approved in 2000. Road signs and textbooks were required to follow these rules as soon as possible, at a cost estimated by the government to be at least US$20 million. Almost all road signs, names of railway and subway stations on line maps and signs etc. have been changed. Romanization of surnames and existing companies' names has been left untouched; the government encourages using the new system for given names and new companies.
- RR is similar to MR, but uses neither diacritics nor apostrophes, which has helped it to gain widespread acceptance on the Internet. In cases of ambiguity, orthographic syllable boundaries may be indicated with a hyphen, although state institutions never seem to make use of this option, e.g. on street signs or linemaps.
- ISO/TR 11941 (1996): This actually is two different standards under one name: one for North Korea (DPRK) and the other for South Korea (ROK). The initial submission to the ISO was based heavily on Yale and was a joint effort between both states, but they could not agree on the final draft. A superficial comparison between the two is available here: 
- Lukoff romanization, developed 1945–47 for his Spoken Korean coursebooks
- Chosŏn Sahoe Kwahagwŏn (Hangul: 조선민주주의인민공화국사회과학원, Hanja: 朝鮮民主主義人民共和國社會科學院) romanization
McCune-Reischauer-based transcriptions and the Revised Romanization differ from each other mainly in the choice of how to represent certain hangul letters. Both attempt to match a word's spelling to how it would be written if it were an English word, so that an English speaker would come as close as possible to its Korean pronunciation by pronouncing it naturally. Hence, the same hangul letter may be represented by different Roman letters, depending on its pronunciation in context. The Yale system, on the other hand, represents each Korean letter by always the same Roman letter(s) context-independently, thus not indicating the hangul letters' context-specific pronunciation.
Even in texts that claim to follow one of the above, aberrations are a common occurrence and a major obstacle, e.g. when conducting an automated search on the Internet, as the searcher must check all possible spelling variants, a considerable list even without such aberrations.
In addition to these systems, many people spell names or other words in an ad hoc manner, producing more variations (e.g. 이/리 (李), which is variously romanized as Lee, Yi, I, or Rhee). For more details, see Wikipedia:Naming conventions (Korean).
SKATS is a transliteration system that does not attempt to use letters of a similar function in Western languages. A similar approach is to transliterate by hitting the keys that would produce a Korean word on a keyboard with 2[du]-beolsik layout. This can often be seen on the internet, for example in usernames.
(RR transcription in brackets)
|“on the wall”||벽에||byeoge
|pyŏge||pyek ey||wsl ktu|
|pakke||pakk ey||well ktu|
|“to the kitchen/in the kitchen”||부엌에||bueoke
|puŏk'e||puekh ey||wh ktx ktu|
|wikibaekkwa||wikhi payk.kwa||khu xu weul lae|
|“Hangul”||한글||hangeul or han-geul
|“(an) easy” (+ noun)||쉬운 …||swiun …
|“Korea has four distinct seasons.”||한국은 네 계절이 뚜렷하다.
(韓國은 네 季節이 뚜렷하다.)
|Hangugeun ne gyejeori tturyeothada.
(Hangug-eun ne gyejeol-i ttulyeoshada.)
|Hangugŭn ne kyejŏri tturyŏthada.||Hānkuk un ney kyeycel i ttwulyes hata.|
|“Just check the line color and width you want.”||원하시는 선 색깔과 굵기에 체크하시면 됩니다.
(願하시는 線 色깔과 굵기에 체크하시면 됩니다.)
|Wonhasineun seon saekkkalgwa gulkgie chekeuhasimyeon doemnida.
(Wonhasineun seon saegkkalgwa gulggie chekeuhasimyeon doebnida.)
|Wŏnhasinŭn sŏn saekkalgwa kulkie ch'ek'ŭhasimyŏn toemnida.||Wēn hasinun sen sayk.kkal kwa kwulk.ki ey cheykhu hasimyen toypnita.|
See also 
- Comparison tables of the different systems:
- Comparison table of ISO TR/11941, North Korean national system (1992), Revised Romanization, McCune-Reischauer, Yale (PDF file from UN Group of Experts on Geographical Names Working Group on Romanization Systems)
- Comparison table of IPA, Yale, McCune-Reischauer, Lukoff, South Korea Ministry of Education, Joseon Gwahagwon, Revised Romanization (PDF file from Glossika Inc.)
- Lukoff's system (simple table)
- Gangmun Romanization
- Online transliteration tool