I know there's kendo, Iaido, and Kenjustu, but that's about as far as my knowledge goes. What makes up the differences between the three? Are they all parts of a well oiled machine? Do the differences have historical beginnings? One of the colleges I'm looking at teaches Kendo, with more of a traditional aspect on it as opposed to sport. I am interested in attending the club at some point if I go there, so it just sparked the curiosity of the differences in the Japanese Sword Arts. Bear with me, I know virtually nothing aside from very basic knowledge of Kendo, (knowing it's "The Way of The Sword.", the difference between a shinai and a bokken, what a bogu is, etc.)
Himura's given a decent overview, so I thought I'd fill in a few details.
To begin with, there's Kendo, Iaido, and Kenjutsu.... and Battojutsu, Iaijutsu, and Iai-nuki. And Koshi no Mawari, and Tachijutsu. And Kogusoku, and Kumiuchi Kenden, Kumitachi, Ryotojutsu, Tojutsu, Nitojutsu, Kodachijutsu, and Gekken. Kenpo, and Kendo (different form). Iai might just be Iai, Ken could just be Ken, and so on as well. Or it all might get classed as Heiho, or Hyoho (different pronunciations of the same thing). So, uh, yeah... there's more to it than just "Kendo, Iaido, and Kenjutsu". But let's take them one by one.
Kendo is, realistically, a modern sporting approach to sword... and, although the literal translation is "Way of the Sword", in some ways, that's about as close as it gets. Due to it's context and methodology, it's actually fairly removed from what might be considered genuine swordsmanship. The targeting has been altered to safer areas, specific to those protected by the armour worn (the bogu), and favouring striking over cutting. As a sporting approach, it's fantastic... but it's not really the same as a true combative approach to sword. In fact, these shortcomings are what has lead to a couple of other aspects. The first is Kendo no Kata.
Kendo no Kata is a series of fundamental kata, 7 for long sword, 3 for short sword, that are taught as part of Kendo. They were put together as a result of a desire for standardization when Kendo (along with Judo) was introduced into schools (around 1911... there was an earlier attempt, in 1906, but that one proved less popular, and didn't survive to be passed on), and aim to teach proper sword handling to Kendo students, being formulated by members of a range of Ryu-ha, with a heavy emphasis on Itto Ryu (a few different forms). As time has gone on, Nihon Kendo no Kata has been more or less ignored by many Kendoka, with the kata only being taught or studied really to satisfy grading requirements, often only for higher Dan rankings. But some teachers do place emphasis on this method, and insist on the kata being studied from early on in the students developments. Odds are that if your prospective club is advertising itself as being more "traditional", this is what their referring to... as pre-WWII Kendo is rather different (what I would consider closer to "traditional Kendo").
Just for fun, here's some old footage... firstly, Kendo no Kata:
Next, some old Kendo footage from the 50's (note the sweeps):
And here's some from 1897 (note the alternate weapons, such as Kusari-Gama, as well as takedowns and grappling):
Kendo's development has been something that's not that easy to follow, though, and here is where it all intertwines with Kenjutsu... in the 17th Century, wars were no longer a common thing... yet Kenjutsu flourished for a range of reasons. Among the various old and new schools of Kenjutsu, a number, such as Kashima Shinryu and (Yagyu) Shinkage Ryu, developed safer training alternates to the established bokken/bokuto (wooden sword). What they came up with was an item called a fukuro shinai... almost literally a "bagged bamboo sword". This was made up with a body of a number of strips of bamboo, covered in a leather casing, and allowed a level of contact to the training not possible with live blades, or even with the hard wood bokken in common usage. As time went on, there were variations to this design being used, and duels (friendly ones, at least) began to be conducted using these safer (and faster) versions. It got to the point where a number of "famous" duelists of this era were known for their specialization in the use of a shinai, often having personally designed or preferred weapons. One in particular gained his notoriety by essentially having a much longer shinai than anyone else, giving him a decisive reach advantage over all others. Interestingly, there were legal restrictions at the time on the size and length of swords that could be worn and used, but not shinai, and it was this loophole that was exploited, primarily to gain fame and followers, with the appearance of a greater skill (due to not being defeated). This didn't last, and the Tokugawa government eventually regulated the lengths of shinai, which was the first step to standardizing what would come to be known as Kendo. It was only by the late 19th Century, after the Meiji Restoration, that what we know as Kendo really started to take shape... but, as seen above, there were still some rather major differences! Many of the rules and regulations that removed the striking, grappling, alternate weaponry and so on, occurred after WWII... although there are some who are trying to bring Kendo back to what it was. You'll be hard pressed to find it outside of certain dojo in Japan, though...
That brings us to the other aspect which was as direct responce to Kendo... Seitei Iaido. Seitei Iaido is, again, a standardized form of an approach to swordsmanship, and was originally developed specifically to give Kendoka some experience and exposure to the way a real sword actually handles, feels, and moves, as well as teaching proper cutting mechanics (rather than the more "striking" approach of shiai-based [competition based] Kendo). It was originally a series of 7 kata, put together in 1969, with another three added in 1981, and two more in 2000, bringing the current list to 12 kata. They are drawn from a small number of classical systems (albeit with small-to-major alterations), such as Muso Jikiden Eishin Ryu/Muso Shinden Ryu, and Hoki Ryu. Although the original intent was more about just giving Kendoka some experience with a sword, as a supplement to their Kendo training, Seitei Iaido has become very much an art taught for it's own sake. This has lead it to a level of, well, some might say anal-retentiveness that goes beyond many other arts... even the Koryu systems they draw their methods from. The level of precision and attention to detail in modern Iaido is incredible, in order to give students the emphasis on refining the self through the refining of the performance of the kata.
Of course, this is modern Iai. There are a large number of classical (Koryu - literally "old styles") arts out there, some of which are just Iai (in various forms), such as Mugai Ryu, Muso Jikiden Eishin Ryu, Muso Shinden Ryu, Tamiya Ryu, Hasegawa Eishin Ryu, Shinmuso Hayashizaki Ryu, and so on, or might be a more holistic approach to martial arts which include an Iai component, such as Tenshinsho Den Katori Shinto Ryu, Kashima Shinryu, Kashima Shinden Jikishinkage Ryu, Tatsumi Ryu, Hontai Yoshin Ryu, Sosuishi Ryu, Araki Ryu, and many, many more. These arts might use the term Iai, Iaido, Iaijutsu, Iai Nuki, Batto, Batto-do, Battojutsu, Batto-ho, Koshi no Mawari, and so on... and might use a variety of terms even within the one system (such as Katori Shinto Ryu, who use the term Iai to refer to their initial methods, which are all seated, as well as their secret methods, which have both seated and standing, and Battojutsu to refer to their standing methods). It is said that the originator, or at least, the person claimed to be responsible for popularizing the term and art of Iai was Hayashizaki Shigenobu, with many established systems of Iai all tracing back to him in one way or another... although, again, there are a range of arts that don't have any connection to him, or even pre-date him.
The only real characteristic that is found in all Iai forms is that the methods begin with the sword sheathed. Most commonly, kata are performed solo (most Japanese arts, whether Kenjutsu, Bojutsu, Jujutsu, or anything else, are passed on by paired kata, rather than solo... in quite a different form to the method of kata found in, say, karate systems), and contain a range of aspects, namely:
Nuki-uchi (drawing and cutting in one action)
Kiri (cutting, once the sword is out)
Chiburi/Chiburui (a ritual movement of the sword, varying from system to system, often erroneously believed to be "shaking the blood off the blade", or cleaning the blade... it's not)
Noto (resheathing the sword).
Additionally, concepts such as Zanshin (awareness after the encounter) are considered vital, just as they are in all Japanese martial arts.
However, these aspects are also not universal... systems such as Sosuishi Ryu don't use Chiburi, they do what is called Chinugui... which is fairly simply wiping the blade clean with a cloth carried for that purpose. Others, such as Tatsumi Ryu, omit anything of the kind, feeling that such actions have little practical value, certainly none as a method of cleaning the blade, so don't bother with it. Then you have arts such as Asayama Ichiden Ryu, who also buck the trend by having all their Iai methods done as a paired kata, instead of solo.
Kenjutsu, on the other hand, is even more varied. Taken simply, it's a combative approach to the use of a Japanese sword... but exactly what that might entail is really up to the system itself. It might just be specializing in single long-sword methods... or might also include short sword, or both at once (or two short swords, in a couple of systems). It might include Jujutsu, or staff weapons, or small weapons as well... and might be geared up to face an armoured opponent (yoroi, Japanese battle armour, not bogu, or Kendo armour), or against unarmoured opponents (suhada)... or might have methods for both. It might include an Iai aspect, or it might not. The opponents might always have a sword, or they might have a variety of other weapons.
To give a potted history, though, the oldest extant art from Japan is believed to be the Maniwa-Nen Ryu, who claim their methods from the 14th Century, and are a source school of many other arts. The Maniwa-Nen Ryu is a branch of the original (no longer existing) Nen Ryu, coming from the Maniwa region, and has a rather unique footwork, giving rise to a very powerful usage of the weapon. The Tenshinsho Den Katori Shinto Ryu, founded in the mid-15th Century, is thought to be the oldest "complete" system... it is what is called a Sogo Bujutsu, or composite system, but centers it's teachings around the sword, believing it to be the most versatile weapon in the warriors arsenal. The late 16th Century saw the rise of a number of arts that trace themselves back to Katori Shinto Ryu, or the arts of the nearby Kashima Shrine, including Kage Ryu, which gave rise to Shinkage Ryu, itself becoming/giving rise to Yagyu Shinkage Ryu. This art gained fame by being known as the teachers of the Tokugawa Shoguns, and Yagyu Munetoshi, as well as his son Munenori, being advisors to the Tokugawa's. There was another art that was taught to them, though, which was Ono-ha Itto Ryu... a much more direct system, branches of which (such as Hokushin Itto Ryu) have been a huge influence on Kendo. Other arts of note are Hyoho Niten Ichi Ryu, the school of Musashi Miyamoto, founded in the mid 17th Century, after a long career of dueling across Japan, and Jigen Ryu, a very intriguing system from Satsuma, known for it's brutal methods, and terrifying practitioners.
So, what's the difference? A hell of a lot. What's the difference between Karate, Judo, and Aikido? Same type of question....