Long before, Buddhism came into Tibet, Tibetans followed the Local Bon religion, a type of Shamanism. which brought from Shang-shung kingdom, Present day Ngari Prefecture. About Bon religion see here detail.
During reign of 28th king Lha thothori Nyantsen, Few Indian missionary came with sacred Buddhist scripture to his court. which nobody understand meaning inside. so keep it secret inside palace, and called it as Nyanpo Sangwang. This is Auspicious symbol of Spreading Buddhism inside Tibet.
By the time of 7th century, Emperor Songtsan Gampo, send his trustworthy scholar Thomi Sambota to study Indian script and language, which he master within few years and came back after six years. He not only recreate Tibetan calligraphy and structural, also translated many of Buddhist scripture, including lotus sutra. That is bold beginning of Buddhism in Tibet as foreign belief and thought.
Songtsan Gambo may be influenced by his wives Trizun Brikuti from Nepal and Wencheng towards Buddhism. but also political and foreign relation reason, he built dozens of Buddhist temple with respective wives. Thus Buddhism rose in popularity in Tibet while Bon went into decline. Buddhism spread quickly, and has exerted an extensive and profound influence on the Tibetan race in terms of Tibetan values, morals, psychology and every aspect of life.
In the late 8th century, in order to shield and promote Buddhism as their state cult, Trisong Deutsan built many monasteries and translated a large number of Buddhist scriptures. He invited Pandit Shantarakshita and Kamalasila from Nalanda university. Later the Padmasambhava, famous Indian Yogis, combined elements from the Indian Esoteric Sect with the Tibetan primitive Bon religion to form Tibetan Tantrism.
An ordained spiritual community was established in the first Buddhist monastery; Samye, which was built by Padmasambhava. In this period, translation of scriptures genuinely began. At that time numbers of Indian scholars invited to Tibet and Buddhism canons were translated into Tibetan. So all these contributed to the firmly establishment of Buddhism in Tibet, as the presence of Sangha is considered essential.
In 792, after a great philosophical debate, King Trisong Detsen officially declared Indian Buddhism to be the state religion of Tibet.
After the strike of Last emperor’s death, Fragmentation of whole Empire into few major kingdoms. Tibetan Buddhism went quiet for more than a century and it began to revive at the beginning of the 11th century.
Atiśa arrived in Tibet at the invitation of a west Tibetan king. This renowned exponent of the Pāla form of Buddhism from the Indian university of Vikramaśīla later moved to central Tibet. It was the prevailing period of Tibetan Buddhism. At the time many different independent sects appeared.
The doctrine of the Tibetan Buddhism is based on the Mādhyamika Doctrine and the Yogācāra, also called Cittamātra “Mind-Only” Doctrine. The general method of practising are Transmission and realization, Analytic meditation and fixation meditation, Devotion to a guru, Skepticism, Preliminary practices and approach to Vajrayāna, Esotericism