Bujinkan is an organization devoted to the study and spread of some schools of Japanese Budo Taijutsu.
Its founder is Hatsumi Masaaki (初見良昭), he is also the grand master of the school (Soke in Japanese terms).
The discipline inherits from nine traditional Japanese martial art schools:
Gyokko-ryū Kosshijutsu (玉虎流骨指術)
Kotō-ryū Koppōjutsu (虎倒流骨法術)
Togakure-ryū Ninpō (戸隠流忍法)
Takagi Yōshin-ryū Jūtaijutsu (高木揚心流柔体術)
Shinden Fudō-ryū Dakentaijutsu (神傳不動流打拳体術)
Kukishin-ryū Dakentaijutsu (九鬼神流打拳体術)
Gikan-ryū Koppōjutsu (義鑑流骨法術)
Gyokushin-ryū Ninpō (玉心流忍法)
Kumogakure-ryū Ninpō (雲隠流忍法)
These are traditional old schools. As an example Togakure-ryū has been tracked to exist since around the year 1161.
The Bujinkan headquarters, known as Hombu Dojo, are located in Noda city. (Chiba prefecture, close to Tokyo)
Hatsumi Sensei teaches there to any practitioners who want to attend.
Nowadays Bujinkan has become an international network with some 150.000 practitioners worldwide.
Bujinkan at CERN
Bujinkan at CERN was founded as a convenient way for its practitioners, working at CERN, to enjoy training in an adequate environment. It soon became self-evident that there was no reason to keep it closed to CERN users/personnel. Nowadays it's open to anyone willing to come.
Bujinkan Budo Taijutsu is a wide discipline. It covers subjects like the correct way to fall, techniques of unarmed fighting, the usage of many Japanese traditional weapons like katanas, tanto knives, different sized sticks or kusari chains amongst many others.
The practice of Budo Taijutsu enhances balance, discipline, self-control and self-confidence. It is a nice way to practice sport with the advantage of having an intellectual background and life philosophy. In the event of unavoidable conflict it can also serve as a very effective self-defence method. And finally, at least for CERN people, it is conveniently close to your job.
What does it look like?
Here is a few selected videos of top level teachers (it doesn't intend to be exhaustive) to illustrate the type of techniques we train:
DO NOT ATTEMPT ANY OF WHAT YOU SEE IN THESE VIDEOS WITHOUT BEING SUPPERVISED BY A QUALIFIED INSTRUCTOR
https://www.youtube.com/watch?v=csMCFHyKW5g (Hatsumi Sensei)
https://www.youtube.com/watch?v=OOP-zyvvD-E (Hatsumi Sensei)
https://www.youtube.com/watch?v=fSwSGJw4eXg (Hatsumi Sensei)
https://www.youtube.com/watch?v=LCUVQzBcZqU (Dai Shihan Paco Roldán)
https://www.youtube.com/watch?v=JD5MvD29tu0 (Dai Shihan Paco Roldán)
https://www.youtube.com/watch?v=kiZMdQN7H4I (Dai Shihan Sveneric Bogsater)
Please note that this list of links to videos is only a sample of publicly available information for example purposes (by April 2016). Bujinkan section does not necessarily support anything that could be said on them and has no knowledge on their origin or legality.