Jejueo cannot be understood by speakers of Korean, but the two languages are nonetheless related to each other, as shown by systematic similarities in their vocabulary and grammar. Together, they make up the Koreanic language family.
Many people born and raised on Jeju Island speak a colloquial variety of Korean that includes some Jejueo as well. It is important not to confuse this with Jejueo itself, which is a genuinely distinct language.
Jejueo [pronounced "jay-joo-uh"] is a language spoken primarily on Jeju Island (33°22'N 126°32'E; population 600,000), one of the nine provinces that make up the Republic of Korea. It is also used to some extent in the Osaka area of Japan by ethnic Koreans who fled there in the late 1940s and early 1950s at a time of harsh military suppression on Jeju Island.
Jejueo has at most 5000 to 10,000 speakers. It is used primarily in informal settings (e.g., in the home), largely by elderly speakers with a strong sense of ethnolinguistic identity. Jejueo was recognized as critically endangered by UNESCO in 2010.