What we study resembles other martial arts in that we gather in a training hall (a Dôjô) to have fun throwing each other around ... but there are a few important differences. For example, our training is not competitive, and is not geared towards "keeping fit" -- it is not a sport; nor is it primarily a "path to enlightenment", although some spiritual aspects may emerge during one's training. As there are no competitions, there are also no rules or limitations on techniques (we study punching, kicking, throwing, joint locks, chokes, nerve points, and just about any conceivable weapon, particularly sticks, swords/knives, halberds/spears, chains/ropes, etc.). This obviously makes the art quite dangerous, but in a good Dôjô the training is always very relaxed and friendly, with a zero rate of injuries (note: I have seen minor injuries in some Dôjô outside Japan, but do not recommend such Dôjô). The training also tends to be very informal, and the different grades & belts do not have such a rigid significance as in some other arts.
Dr Hatsumi has inherited nine schools of martial arts: Togakure-ryû, Kotô-ryû, Gyokko-ryû, Takagi Yôshin-ryû, Shinden Fudô-ryû, Kukishin-ryû, Kumogakure-ryû, Gyokushin-ryû, and Gikan-ryû. If you are interested in the differences between the schools, the history of our art, etc. I suggest you follow some of the Bujinkan links on this site. However, please remember that much of this type of "information" does not originate from Dr Hatsumi (and even that which does is often misinterpreted en route). Look on it like a historical novel: it may be interesting, sound reasonable and even be based on fact but it's best not to use as a textbook unless you conduct some research of your own.