Tibetan Buddhism is often called Lamaism because of it's prominent lama tradition. "Lama" actually means Highest Mother, even though lamas most of the time are male. They ar so called because they have reached a level of insight that makes them think of all living beings as a mother loves her child unconditionally.
There are four schools of Tibetan Buddhism. Nyingmapa, Kagyupa, Sakyapa og Gelugpa.
Nyingmapa is the second biggest school and the oldest. They consider Guru Rinpoche "the second buddha". He merged buddhism and the Bön religion that already existed in Tibet when Buddhism was brought there. The distinctive doctrine of the Nyingma school is Dzogchen ("great perfection"), also known as ati-yoga (extraordinary yoga). It also makes wide use of shamanistic practices and local divinities borrowed from the indigenous, pre-Buddhist Bon religion. Nyingma monks are not generally required to be celibate.
Kagyupa's ("oral transmission school") teachings were brought to Tibet by Marpa in the 11th century. As its name indicates, this school of Tibetan Buddhism places particular value on the transmission of teachings from teacher to disciple. It also stresses the more severe practices of Hatha Yoga. The central teaching is the "great seal" (mahamudra), which is a realization of emptiness, freedom from samsara and the inspearability of these two.
Sakyapa is the smalles of the four. The abbots were devoted to the transmission of a cycle of Vajrayana teachings called "path and goal" (Lamdre), the systemization of Tantric teachings, and Buddhist logic.
Gelugpa is the biggest and the newest version of buddhism that came to Tibet. It is the school of Dalai Lama, and they are often called "yellow hat school". It was founded in early 14th century and it restored celibacy and prohibition of alcohol and meat, enforced strict monastic discipline and established a higher standard of learning for monks.
So what is special for the Tibetan branch of Buddhism? In common with Mahayana schools, Tibetan Buddhism includes a pantheon of Buddhas, bodhisattvas, and Dharma protectors. Arya-bodhisattvas are able to escape the cycle of death and rebirth but compassionately choose to remain in this world to assist others in reaching nirvana or buddhahood. Dharma protectors are mythic figures incorporated into Tibetan Buddhism from various sources (including the native Bön religion, and Hinduism) who are pledged to protecting and upholding the Dharma. Many of the specific figures are unique to Tibet.
(information from : http://www.religionfacts.com/buddhism/sects/tibetan.htm)