Kyokushin kaikan (極真会館?) is a style of stand-up, full contact karate, founded in 1964 by Korean-Japanese karate master, Masutatsu Oyama (大山倍達, Ōyama Masutatsu?) who was born under the name Choi Young-Eui (최영의). Kyokushinkai is Japanese for "the society of the ultimate truth." Kyokushin rooted in a philosophy of self-improvement, discipline and hard training. Its full contact style has had international appeal (practitioners have over the last 40+ years numbered more than 12 million[1][2][3]).
Kyokushin has influenced many of the " Full-Contact" schools of karate, emphasizing realistic combat, physical toughness, and practicality in its training curriculum. The Knockdown karate competition system pioneered by kyokushin has been adopted by many different karate styles and organizations. Other martial arts organizations have "spun off" from Kyokushin over the years, with some adding additions, such as grappling techniques, but continuing with the same philosophy of realistic and practical training methodals.

Origin
The following is a brief overview of the early life of Masutatsu "Mas" Oyama.
The founder of Kyokushin, Masutatsu Oyama,he was born Choi Yong-i on 27 July 1923 in Gimje, Korea[4], during the long period of Japanese occupation. Owing to the popularity of some recent movies, the name "Choi Bae Dal" is sometimes used to refer to Oyama. 'Bae Dal' is the Korean pronunciation of his Japanese name 'Masutatsu', which can also be pronounce 'baitatsu'. However Oyama himself never used this name and its use is a more recent introduction.
As a young child, Oyama enjoyed fighting and watching others fight. In 1938, he emigrated to Japan and studied Okinawan Karate under Gichin Funakoshi, eventually gaining 2nd dan. Later, Oyama also trained under Yoshida Kotaro, a famous Daito-ryu Aiki-jujutsu/Yanagi-ryu Aiki-jujutsu master, from whom he received his menkyo kaiden - an older form of grade, a scroll signifying mastery, from Kotaro. This scroll is still on display at the Honbu dojo in Tokyo.
Also, upon the advice of his mentor and a member of the National Diet, Matsuhei Mori, around this time the young master took his Japanese name, Masutatsu Oyama, the name he would use for the rest of his life. After World War II, Oyama began his training in Goju Ryu karate under a Korean master in Japan, So Nei Chu, who ran a dojo in Tokyo with the famous goju teacher Gogen Yamaguchi. He would finally attain 8th Dan in Goju Ryu Karate. Another influence from the Goju school was Masahiko Kimura, although also an assistant karate instructor at the karate dojo Oyama trained at, Kimura was primarily a famous champion of judo who defeated Hélio Gracie of Brazilian Jiujitsu fame. Kimura encouraged Oyama to take up judo so that he would have an understanding of the art's ground techniques. Kimura introduced Oyama to the Sone Dojo in Nakano, Tokyo, where he trained regularly for four years, eventually gaining his 4th Dan.
It was after this time that Oyama first retreated into the mountains for one of his well-known solitary training periods, yamagomori. He completed two such retreats for a total of almost three years of solitary training in accord with the ascetic traditions of many of the great warriors of Japan through the centuries. During this period of isolated training, Oyama engaged in intense shugyo, or spiritual discipline.
In the early 1950s, Oyama traveled to the USA visiting 32 states

In 1953, Oyama resigned from Goju ryu and opened his own independent karate dojo, named "Oyama Dojo" in Tokyo, but continued to travel around Japan and the world giving martial arts demonstrations, including bare-hand challenges.[citation needed] His first 'Oyama dojo' was a vacant lot in Mejiro, Tokyo. In 1956, Oyama moved the dojo into the ballet studio attached to Rikkyo University. Oyama's own curriculum soon developed a reputation as a tough, intense, hard-hitting, and practical style which he named "Kyokushin" in a ceremony in 1957. As the reputation of the dojo grew, students were attracted to come to train there from Japan and beyond and numbers grew.
In 1964, Oyama moved the dojo into a building he refurbished not far from the ballet studio at Rikkyo. Oyama also formally founded the "International Karate Organization Kyokushinkaikan" (commonly abbreviated to IKO or IKOK), to organize the many schools that were by then teaching the Kyokushin style. This dojo at 3-3-9 Nishi-Ikebukuro, in the Toshima area of Tokyo, remains the world headquarters of the Kyokushinkaikan to this day, although there are a number of breakaways that claim authority but are not recognized legally in Japan. All enquiries regarding Mas Oyama's Kyokushin Karate should be directed to this Honbu.

1964 to 1994
After formally establishing the Kyokushinkaikan, Oyama directed the organization through a period of expansion. Oyama hand-picked instructors who displayed ability in marketing the style and gaining new members. Oyama would choose an instructor to open styles in another town or city in Japan. The instructor would move to that town and usually demonstrate his karate skills in public places, such as at the civic gymnasium, the local police gym (where many judo students would practice), a local park, or conduct martial arts demonstrations at local festivals or school events. In this way, the instructor would soon gain students for his new dojo. After that, word of mouth would spread through the local area until the dojo had a dedicated core of students. Oyama also sent instructors to other countries such as the Netherlands (Kenji Kurosaki), Australia (Shigeo Kato), the United States of America (Tadashi Nakamura, Shigeru Oyama and Yasuhiko Oyama, Miyuki Miura) and Brazil (Seiji Isobe) to spread Kyokushin in the same way. In addition, numerous students began to travel to Japan to train with Oyama, consequently returning to their country to spread the art. In 1969, Oyama staged The First All-Japan Full Contact Karate Open Championships which took Japan by storm and Terutomo Yamazaki became the first champion. All-Japan Championships have been held at every year. Also in 1975, The First World Full Contact Karate Open Championships were held in Tokyo. World Championships have been held at four-yearly intervals since, although under the current confusion of self-proclaimed representative organizations, there are up to five so-called "world championships" a year, a situation which no longer makes any sense. Especially to the fighters who, in the past, knew who the Kyokushin world champion was at any time. Now we have a situation where there are Kyokushin "world" champions who won their title in obscure tournaments that are not at all representative of Mas Oyama's Kyokushin Karate.
Divided organization since 1995
Upon Oyama's death, several self-made groups broke away from the International Karate Organization (IKO) Honbu, primarily due to personal conflicts over who should succeed Oyama as Chairman. As a supposed will was proven to be invalid in the family Court of Tokyo in 1994, any claim to that will indicating the true intention of Oyama was nullified. Before his death, Oyama named no one as his successor although many now claim to be the rightful leader of his organization.
The only organization which is legally recognised as related to Oyama's original Kyokushin Honbu Dojo (school) and Oyama's family (who inherited Oyama's intellectual property) is the IKO Kyokushinkaikan based at the original dojo of Oyama, identified as the IKO Sosai (see [2]).
Kyokushin today
Existing as a single organization under the leadership of the founder, Mas Oyama, the Kyokushin organization was damaged, perhaps beyond repair, by numerous breakaway groups who left the IKO after the Master's passing. Each claim their own authority as representing the original Honbu, although legally the only organization recognized in Japan as having authority to use the intellectual property of Mas Oyama and Kyokushin Karate is the original IKO organization based at Oyama's original dojo. Many of these groups use identical names or minor variations thereof. As a result there is much confusion and political (and legal) rivalry between the groups, exacerbated by the ongoing claims by the breakaway groups that they are legitimate, despite legal direction to cease.
Oyama's widow died in June 2006 after a long illness. According to the Japanese legal system the Custodian of Oyama's intellectual property and legacy is the youngest of his daughters, Kikuko (also known as Kuristina) who now oversees the management of original IKO Honbu, although not directly involved in karate teaching.

The source Wikipedia