Between the 7th and 9th centuries, the Tibetan empire was a dominant power in Asia. Tibet had wide authority throughout the Tarim region, which extended into Northern China and Nepal. During the reign of King Songtsen Gampo (born appx. 557 C.E.), a Tibetan script was created which permitted the translation of Buddhist Sanskrit texts into Tibetan. Many great translators over the following centuries performed the amazing feat of translating almost the entire corpus of Indian Buddhist texts into Tibetan.
In the 8th century, under King Trisong Detsen, a wave of great Buddhist teachers traveled to Tibet from India. Incomparable masters such as Padmasambhava, Vairocana, Santaraksita and Vimalamitra spread the dharma throughout Tibet. The great monastic university of Samye was founded during at this time, and Buddhism became well established in Tibet.
A few decades later, Buddhist practice in Tibet met with strong indigenous Tibetan opposition, and Buddhist practitioners were no longer supported by the government. However, Buddhist institutions gradually recovered and a second wave of translations and teachers came to India in 11th and 12th centuries.
Tibetan Buddhism can be divided into numerous "schools." These schools are doctrinally very similar, but are distinguished by the different teachers and texts considered authoritative in each school. Living Buddhist traditions are transmitted through these teachers and texts and institutions. In Buddhism, this transmission is known as "lineage."
The basic outward mechanism of Buddhist lineage is the teacher-student relationship. A particular teacher teaches his student(s) through oral means, and by selecting and explaining the meaning of certain authoritative Buddhist texts. Such oral teachings, as well as any accompanying textual material, is passed by that student to his own student(s). Ideally, this outward process of exchange between teacher and student carries with it an inner process whereby the spiritual realization of the teacher is also passed from teacher to student. It is indeed the transmission of this living quality of wisdom which is the essential quality of Tibetan Buddhism today.
Traditionally within Tibet there are many groupings of schools, such as the Eight Chariots of the Practice Lineage. However, the most well-known grouping in modern times is into the four major schools: Nyingma, Sakya, Kagyu and Gelugpa.
The Sakya, Kagyu and Gelugpa are "new schools," established during the second wave of Buddhism's transmission from India to Tibet in the 11th and 12th centuries.
In general, the lineage traditions are supported by different, independent Tibetan institutions. Each of these major schools were predominant in Tibet during different periods of Tibetan history, and each have geographical areas where they are more popular. There are also innumerable subschools and divisions within each of these four major schools, a tendency which is heightened by the individual nature of lineage transmission. Still, many unifying movements have informed Tibetan institutions over the centuries. In particular, in the Nineteenth Century, the Rime movement led by Jamgon Kongtrul the Great and the great Jamyang Khyentse, revitalized many Sakya, Nyingma and Kagyu institutions by promoting respect and knowledge by each of the other.