The simplest division of the different forms of Buddhism is between the oldest surviving school of Theravada (“the Teachings of the Elders”) which predominates in Southeast Asia and Sri Lanka and Mahayana (“Great Vehicle”) in East Asia. Most Buddhists will identify themselves as either Theravada or Mahayana. Scholars generally divide Mahayana Buddhism into East Asian (or plain Mahayana) and Vajrayana (sometimes translated as “Diamond Vehicle” or “Adamantine Vehicle”) Buddhism which is frequently used as an interchangeably with Tibetan Buddhism although Vajrayana also properly includes the Japanese school of Shingon. Hinayana, “Lesser Vehicle”, is another term sometimes used as a synonym for Theravada which is may be considered derogatory. Hinayana is also sometimes used to refer to the early Buddhist schools existing prior to the Mahayana school splitting off.
The Original Split
Information about the original splits between types of Buddhism is somewhat muddy. Traditions hold that early Buddhism had split into the probably apocryphal number of 18 schools by the time of the Third Buddhist council around 250 BCE. Of the 18 schools only Theravada, then known as Sthavira, survived beyond the end of the medieval period although some Mahayana doctrines have discernible roots in the early schools of Mahasanghika and Sarvastivada. Mahayana’s origins are even murkier, being dated to either the 1st century BCE or the 1st century CE in south, south-east or north-west India. The most likely reason for the Mahayana split is Vinaya, monastic discipline, as early Mahayana literature idealized the lives of ascetic forest monks. By the time Mahayana was fully established in 5th century CE India it featured settled, landed monasteries integrated with lay communities which appears to have been what the school originally most loathed.
Theravada is a form of Buddhism that tends to be relatively orthodox and conservative. Theravada philosophy revolves around Vibhajjavada, the “Teaching of Analysis”. The seeker of enlightenment must use their own experiences and analysis thereof while heeding the body of accepted wisdom. Human existence and suffering are caused by cravings which are accompanied by a formally described system of kilesas (defilements). Degrees of defilement flux in and out of transitory being and are harmful to the self and others. Defilements arise from ignorance and sensation. The state of deep concentration known as jhana allows the senses and defilements to be suppressed, allowing the mind the opportunity become undistorted and investigate reality. Theravada teaches that the path to enlightenment lies in first restraining the defilements through mindfulness, then uprooting them permanently through the internal process of analysis and experience in jhana which leads the meditator to realize the Four Noble Truths and attain enlightenment, Nirvana and freedom from rebirth. Personal experience, not mere belief, of the truths of Theravada is considered essential to attaining enlightenment.
An interesting form of monasticism within Theravada is the Thai Forest Tradition. Wilderness is used as a setting for meditation and strict adherence to Vinaya, seeking to emulate the often forest dwelling lifestyle of the original Buddha and his disciples. The Buddha, in the Pali Canon discourses, directed his disciples to dwell in seclusion in such places as forests, caves, glens, out in the open, or beneath a tree. Ironically enough, this Theravadan tradition, which was revived in the early 20th century in response to increasing urbanization, strongly mirrors what may have been the original motivations for the Mahayana split. Forest monasteries are located as remotely from urban areas and monastic practice is heavily focused on meditation. It is common for young Thai men to temporarily ordain as monks for the July-to-October rainy season retreat of Vassa. Cambodia, Myanmar and Sri Lanka have similar monastic forest traditions.
Mahayana is the largest and most diverse of the different forms of Buddhism. Some of the varieties of Mahayana are Pure Land, Zen, Tendai, Nichiren, and arguably Vajrayana. One of the few constants is an emphasis on the goal of liberation from suffering of all beings vs the Theravadan goal of individual freedom from rebirth. This is the “Great Vehicle” of Mahayana, ultimately bearing all to Nirvana, vs derogatorily referring to Theravada as Hinayana, “Lesser Vehicle”. Mahayana schools usually believe in a pantheon of god-like bodhisattvas who (are usually thought to) have held back from attaining full Buddha-hood out of devotion to the liberation of all sentient beings from suffering and that everyone will eventually become a Buddha. In Mahayana the Buddha is seen as an at once immanent and transcendent all-pervasive principle vs the original Buddha being viewed only as a teacher in Theravada.
Compassion, which is important in all types of Buddhism, is especially stressed in Mahayana and linked to the idea of the transfer of achieved merit, tying in nicely with the goal of the liberation of all sentient beings. Another Mahayana concept is “skillful means”, a “whatever works” attitude to pointing the way to liberation. Another Mahayana concept is that of seeking salvation by way of rebirth into paradise realms ruled by Buddhas and bodhisattvas where it is easy and pleasant to attain Buddhahood. Rebirth into paradise is the cause of the mass popularity of Pure Land form of Mahayana Buddhism, which is heavily devotional in practice. In Zen, another Mahayana variant, the Bodhisattva pantheon is regarded as aspects of consciousness within a central focus on meditation.
Scriptures are plentiful in Mahayana Buddhism and the Lotus Sutra, which speaks of infinite Buddhas and the universal possibility of Buddhahood, is one of the most read and revered in Mahayana practice. Those forms of Mahayana Buddhism that seek salvation through rebirth into paradise have are primarily focused on devotion to Buddhas and bodhisattvas. Meditation is, of course, an enormously important Mahayana practice.
Vajrayana Buddhism likely arose in the 6th and 7th century CE. Vajrayana is also known as Esoteric Buddhism and Tantric Buddhism and is unique among the different types of Buddhism for the prominence of ritual, which often replaces meditation. The “vajra” is a thunderbolt made of the indestructible adamantine substance, therefore Vajrayana may be translated as “Adamantine Vehicle” or “Diamond Vehicle”. Within the Tantric scriptures Vajrayana is sometimes seen as an advanced, “fruitional” vehicle for which Mahayana is the preparation. This notion of Vajrayana as the ultimate vehicle is not held within Mahayana schools that do not overlap with Vajrayana. Vajrayana is a path in which the practitioner is said to be able to reach enlightenment in one lifetime, but is also considered riskier.
Vajrayana involves a vast body of lineages of esoteric knowledge which must be transmitted by initiation from a guru and enables the practitioner to experience Buddha-nature before attaining enlightenment. Ritual is the primary practice where meditation would be central in Theravada and Mahayana. Like Mahayana Buddhists, practitioners of Vajrayana seek to become bodhisattvas. Samaya, tantric vows, are generally taken at the various levels of initiation and it is particularly important that gurus abide by their vows.
Tibetan, Shingon and Tendai schools of Buddhism are both Vajrayana and Mahayana. In Tibetan Buddhism Mahayana is the most usual path with Vajrayana practices of Tantra, Mahamudra and Dzogchen as “skillful means” reserved for advanced students. The Tendai school of China and Japan considers Vajrayana rituals to be of equal importance with the Lotus Sutra. Shingon, a Japanese school reputed to have vast magical powers, has a very few exoteric Mahayana practices for the layperson and holds everything else within the bonds of initiation.
There are also forms of Buddhism which are thoroughly crossbred with other religions. Hinduism is probably the largest influence across the different types of Buddhism in general and has particularly strong fingerprints in Vajrayana Buddhism, Tantrism being as much Hindu as Buddhist. Zen, which is said to originate in a transmission that did “not stand upon words”, is a Chinese-originating hybrid with Taoism. Dzogchen, generally considered to the be highest Vajrayana practice in Tibetan Buddhism, is equally a part of the shamanic pre-Buddhist Tibetan religion of Bon.
Some useful Wikipedia pages:
Early Buddhist Schools
Thai Forest Tradition