The martial art known as Ninjutsu is very secret, with a very clouded history. The history isn't documented all that well, as most of what is known about the martial art is that which has been passed down from generation to generation. There are a lot of historical records that state families from the Koga region as being the creators to this very secret style.
Over 70 different Ninjutsu Ryu have been identified and discovered over the years, however most of them have died out. A majority of them were created around a set of specific techniques and skills, although when those skills of a specific Ryu were no longer needed or wanted, the Ryu seems to die out and fade away from existence.
The art of Ninjutsu is best associated with the ninja's from ancient Japan. The ninja's are well known all around the world, for their stealth and very secreted life. The ninja is known to have went through very tough and demanding training, which hardly anyone really knows about. Those that were actual ninja's are either dead, or not allowed to let anyone know their true identity.
During the 1980's, when the ninja trend really hit the United States, the instructors of Ninjutsu popped out all over the place, making black a very happening color. Years later, the trend seemed to die out, and there aren't that many people trying to learn the art.
In regards to the art, the term of Ninjutsu doesn't actually refer to any certain style, but rather a group of the martial arts, with each one having it's own view that's expressed with each of the different Ryu. The Ryu vary, meaning that one may focus on one certain physical dynamic, while the next may be focused on redirection.
What many people aren't aware of, is the fact that Ninjutsu involves both unarmed and armed fighting skills, along with philosophy, strategy, and history. There are a few dojo's that offer the art, many of them being quite comprehensive with the way they teach.
During combat, Ninjutsu focuses on distance, posture, and flow. Stylists are taught to react to every movement, and respond in ways that will place them in the advantage position. From being in a position of advantage, the stylist can effectively change the outcome of the encounter - quickly going from negative to positive.