Ryu, also romanized as 'ryuu' is described suscinctly in David Lowry's Sword and Brush as "The Tradition." It is also referred to as "The Flow," as the kanji can also be pronounced as 'nagareru,' a verb also meaning 'to flow.' It is used in the formal title of an art to denote the particular style practiced. An example is Muso Jikiden Eishin Ryu.
Ha, means faction, group, or sect. It is used in the formal title of an art to denote the branch of a style, a tributary to the great flow. An example is Toda-ha Buko Ryu.
The concept of the ryu-ha is one that is uniquely Japanese. Not because other countries were incapable or didn't set up structured institutions to pass on their cultural traditions, but because of the all-encompassing nature of the ryu-ha. Outer, omote, traditions and stylistic ways make a ryu-ha visible and distinguishable to even the untrained, but it is the inner, ura, nature of the ryu-ha that truly defines it. It is the principle, the outlook on life, that is taught and makes the practitioner what he is. In a way, the concept of ryu-ha is only synonymous with the German concept of umvelt.
The ryu-ha is perpetuated through a student-teacher relationship known as direct transmission. This is akin to the Doctorate of Philosophy system, whereby a teacher takes particular interest in a single or small number of students and directly passes a great deal of his knowledge to this student through close contact and an intimate relationship. The leadership of the ryu-ha is thus nurtured from one generation to another, flowing directly from one man to another, as to maintain integrity.
In many cases, the ryu-ha is a proprietary entity, often held closely to a family or prefecture. There are many documented cases of senior students being adopted to allow for transmission when a direct descendent is unwilling or unavailable.
Ryu-ha in the martial arts
During the Muromachi era and on through the pax Tokugawa shogunate, the warmaking in Japan slowed sufficiently enough to allow the warrior reflection on his craft. The samurai, or bushi, had a chance to collect, organize, and thus disseminate those principles and techniques which had led to his success and his continued relationship with life. These principles, either through deliberate construction, or a successful lineage came to be ryu-ha.
The description of a school or style as a flow is fitting. The thought process gained by the thorough study of a martial principle is passed from one generation on to another. The information is flowing through time and warriors. Techniques also changed over time. What was effective in one age might not be in another. Weapons were added or removed. Complete schools were disintegrated or subsumed. In this respect the ideas flowed.
Ryu-ha in other arts
Martial arts are not the only extant examples of the ryu-ha structure. Arts such as ikebana (flower arrangement), chado (tea ceremony), and shado (calligraphy) all participate in the ryu-ha methodology.