Jodo is the way of the Jo, or wooden staff. The most practiced form of Jodo today is the training set adopted by the Zen Nihon Kendo Renmei known as Zen-Ken-Ren Jodo. (formally Seitei Jodo).
While developed in the late 1960's, Jodo is derived from the jo techniques of Shindo Muso Ryu. This koryu or old tradition, which includes a variety of weapons (jo, ken, kodachi, nito, tanjo, jutte, tessen, kusarigami, ...) was founded around the end of the 16th century by Muso Gonnosuke (Katsuyoshi), a student of Katori Shinto Ryu and Jikishinkage Ryu.
Very little concrete information is known about the life of this person. Most accounts associate his invention of the jo-sized staff techniques with an encounter and a subsequent vision. The encounter was with the famous swordsman Miyamoto Musashi (Shinmen Musashi No Kami Fujiwara No Genshin), founder of Hyoho Niten Ichi Ryu and author of the Gorinnosho (the book of five circles).
Much legend has accumulated around these men and their encounter, so we will likely never find out what actually happened. One of the less plausible but most often repeated stories has Muso and Musashi meeting for a duel, the first with a long (six shaku) bo, the latter with his trademark two swords. Muso swings to Musashi's head, who blocks in jujidome (an overhead x-block).
"Kusou, why didn't I think of that?" says Muso, and he goes off to sulk, but suddenly gets an idea. He shortens his stick and hurries back to Musashi. "Watch this, Miyamoto!" he shouts as he swings again to Musashi's head, who obligingly blocks again in jujidome.
However, thanks to the shorter stick, Muso is able to hit Musashi in the plexus with the other end. "Got ya! Bwahahahaaa!" And thus a new koryu was born.
Unfortunately for this interesting story, two of the oldest accounts of the meeting between Muso Gonnosuke and Miyamoto Musashi have Gonnosuke attacking with a wooden sword, not a staff, and Musashi defending himself, not with two swords, but with a short twig he was carving into a toy bow at the time.
The cryptic vision that told Muso Gonnosuke to be mindful of Suigetsu, was (according to my pure speculation) much more likely to refer to Suigetsu the principle rather than Suigetsu the vital point (plexus). The principle of Suigetsu (water-moon) refers to becoming one with the opponent; like the reflection of the moon follows the movements of the water without resistance, yet without being carried along. This principle is closely associated with sen or initiative.
By becoming one with the opponent, the martial artist dominates the encounter. Ueshiba Morihei described the essence of technique as (my rendition):
"Envelop your attacker with your spirit, then you will foresee all his actions and be able to direct him where you wish. By leaving yourself open, you can force him to attack. When you have received 99% of the attack and are already across the border between life and death, only then will you clearly see the direction to evade. This is how you must train. Like this, the ancients could gain insight in military strategy, even training on a single tatami. This is the principle of Suigetsu." (Budo renshu)
Anyone who has gotten some serious Jodo instruction, will be able to symphatise with Ueshiba's feeling of having to cross the border of life and death in order to receive a technique...
According to other stories, Gonnosuke concluded that swords hurt people but sticks would not. To develop this idea, he confined himself to a holy temple, Homangu, the training centre of the ascetic mountain Sect of Tendai, where he received the revelation on the true nature of the round stick. Most traditional Japanese weapons are meant to kill or to cause serious injury. One can, however, use a jo to subdue an opponent without causing permanent damage, while still remaining capable of dealing with more "deadly" weapons, and overcoming them if need be. The jo can also be used to lethal effect, so there's a great degree of flexibility in response. Common jo techniques are thrusts against the face or the plexus solaris, and overhead blows against opponents' head or hands.
Gonnosuke is said to have combined swordsmanship and the art of the yari and naginata into Shindo Muso Ryu Jojutsu.
The sticks have a diameter of 8 bu (2.42 cm), and a length of 4 shaku, 2 sun and 1 bu (127.6 cm). One can strike and thrust using both ends. A flexible length can be obtained by sliding the jo and by changing the position of the hands.
Because these are "real" weapons and very dangerous if used improperly, the jo doesn't have a training method or competitive aspect where one uses protective equipment and engages in freestyle sparring with a mock weapon as in kendo and naginata.
One trains only through the use of kata (pre-arranged formal training exercises). This may sound a bit dull, but it is certainly an effective method for learning the essential elements of distance, timing, trajectory, and awareness.
After developing his art, Gonnosuke became the teacher of the Kuroda clan, and Jojutsu was one of the most secretive arts of the period. Permission to teach outside the han (feudal territory) was not granted until 1872 after the Meiji Restoration had taken place. In the 20th century Takauki Shimizu changed the name of the style to Jodo, and developed the 12 kihon (basic exercises) as a training aid.
Jodo Kyohan by Kaminoda Tsunemori and Nakashima Asakichi.
Edited by Shimizu Takatsugu. This is an almost complete manual of Shinto Muso Ryu Jodo. It is 352 pages long, and has over 2000 photographs, detailing all of the techniques up to Menkyo (no tanjo, jutte, etc. though).
Jodo Kyoten by Yoneno Kotaro and Hiroi Tsunetsugu
Yoneno Sensei and Hiroi Sensei are both 9th dan Hanshi of the All Japan Kendo Federation. Their book shows them demonstrating every part of the All Japan Kendo Federation Jodo Seitei Kata. It starts with the basic postures for holding the jo, and then moves through the basic solo techniques, paired practice of the basic techniques, and finally the Jodo Seitei Kata. Yoneno Sensei and Hiroi Sensei personally posed for all of the pictures in this excellent book.
Jodo - The way of the stick/La voie du baton by Pascal Krieger
Detailed info about Shindo Muso Ryu, but also includes general info about Budo and Japanese history; in English and French