Jiu Jitsu
Jiu-Jitsu is the martial art of the Samurai, as well as being Japans founding martial art. Therefore the history of Jiu Jitsu pre-dates written records going in to legend. Jiu Jitsu is therefore the father of Japanese martial arts. It's techniques historically came from use of the katana, unarmed techniques were developed for the warrior who was disarmed in combat, faced with an armed opponent. Jiu-Jitsu is a system of fighting, it encompases martial art spirit and ritual only when combined with certain aspects of bushido (the way of the warrior). Incidentally, Jiu Jitsu is the only true martial art to be described in this node.
Karate
Karate is not strictly Japanese; its origins lie in Okinawa, an island between China and Japan. Originally China invaded, latter Japan followed and as before the Japanese placed a ban on weapons in 1314.
Okinawan weapon system largely developed under oppression; as such they were developed covertly. Some examples are as follows:
Kai - Boat oar
Kama - Sickle
Tonfa - Rice grinder handle
Knunchaku - Rice flail
Bo - Staff (Originally from China).
Latter these weapons would be incorporated in to Japanese Jiu-Jitsu styles.
Judo
Jigaro Kano Kodokan, creating the first Japanese ′way′ by founded judo in 1882. A way or do is a derivative of the ′old style′ Jiu Jitsu. A Do is supposed to be a way of living - a complete holistic system i.e. a way, some say that Judo's exadurated competition element detracts from its original goal. All old styles were banned during the Meji restoration; Jitsu was again banned again during WW2. This meant that Judo which was classed as a sport was allowed to flourish like a phoenix from the flames.
Aikido
Aikido was founded in 1938 by Ushiba Morihei, which was based on Aiki-Jitsu. Aiki-Jitsu had been considered to be of pure Japanese origin, with any outside influence largely removed. Aikido technique and movement is based on the sword, primarily using circular movement. Aikido could be translated as the way of harmony and spirit.