The budo of Japan are defined as the "martial arts" of Japan. The term, more properly and literally defined as "martial ways," in its general usage, applies to all arts of a martial nature, whether or not they were developed by the classical hereditary warrior caste, or by commoners. The term can be used in a more limited and structured fashion to mean only what Japanese call "gendai" budo, or modern budo. Gendai budo refers to budo that became finalized after the Meiji restoration (1868), in other words, budo forms that were established only one or at most three generations ago. In that case, martial arts systems that were developed prior to 1868 are referred to bujutsu (martial methods or arts).

The definition is not all that concrete, at least not in Japan. Many bujutsu teachers call their arts a "budo," for the sake of convenience or some other reason. In addition, budo offers an emphasis on the spiritual and meditative aspects of a martial art, as the "-do" comes from the Chinese character for the Taoist "Way" (tao or in Japanese -do). To practice a martial "way" is to emphasize the spiritual aspect of the art as an end point to mental and physical training.

Another set of terms for budo and bujutsu, which are more concrete, are gendai budo and koryu budo/bujutsu. Gendai budo, as previously mentioned, means "modern" martial arts, usually of a large-audience appeal, established in "modern" (gendai) times. Koryu ("Old Tradition") are martial arts schools that are older than 1868.

The Ryu

All koryu and several gendai budo schools are called such-and-such-ryu. The -ryu appended to a title refers to the system's "name"; it is a "style" or "school" description. Therefore, karatedo, while a united gendai budo fighting system originating out of Okinawa, retains several strains that are quite different in their kata and training methods. A Shotokan stylist would be different from the Goju-ryu stylist, as opposed to a Uechi-ryu stylist. They would all be doing karatedo, certainly, but even the application of a simple punch could be very different in terms of placement of the arms, use of force, and so on.

Some gendai budo do not have any -ryu systems. Kendo, for example, has been unified as a national (and international) sport and pastime for the use of bamboo staves to represent sword fighting. There are no "styles" or schools of kendo. There is only kendo. All kendo schools share the same general rules when it comes to competition and kendo kata (forms). Likewise judo, although it has fragmented a lot in recent times, is basically judo. Even recent iterations of some modern "jujutsu" schools are, at heart, simply subsets of some parts of judo, emphasizing perhaps the more combative or roughhouse aspects of judo to the loss of the sportive aspects.

Aikido started out as one and only one martial art form. It, too, has developed along different stylistic lines due to differences of style, personality clashes, and other kinds of martial arts politics. Aikido arts presently encompass various different schools, the larger ones being Aikikai (or hombu, the "main branch" style of aikido), Ki Society (Ki No Kenkyukai), Yoshinkan, and Tomiki-ryu.

The differences between koryu schools are even wider. While a Shito-ryu stylist may find similarities in a kata performed by a Shotokan stylist, some jujutsu schools, for example, have no common grounds with each other. Even similar techniques may go by completely different names.

Koryu

The number of distinct koryu schools has decreased since Japan's modern era (1868), due to Westernization, the consolidation of martial activities into the modern -do schools, and sheer neglect. However, a number of koryu schools continue to this day. While koryu budo once numbered in the thousands, they now number in the low hundreds, if at all that much.