Vajrayāna Buddhism is often viewed as the third major 'vehicle' (Yana) of Buddhism, alongside Theravada and Mahayana. The Vajrayana is an extension of Mahayana Buddhism consisting not of philosophical differences, but rather the adoption of additional techniques (upaya, or 'skilful means').
Vajrayana exists today in the form of two major sub-schools:
1Tibetan Buddhism, found in Tibet, Bhutan, northern India, Nepal, southwestern and northern China, Mongolia and, various constituent republics of Russia that are adjacent to the area, such as: Amur Oblast, Buryatia, Chita Oblast, Tuva Republic, and Khabarovsk Krai. There is also Kalmykia, another constituent republic of Russia that is the only Buddhist region in Europe, located in the north Caucasus. While Vajrayana Buddhism is a part of Tibetan Buddhism (in that it forms a core part of every major Tibetan Buddhist school), it is not identical with it; Tibetan Buddhism also includes other vehicles; in fact, many sects of Tibetan Buddhism teach mainly ordinary Mahayana teachings to beginners and laypeople and keep Vajrayana teachings for initiates. Vajrayana in Tibetan Buddhism, properly speaking, refers to tantra, Dzogchen (mahasandhi), and Chagchen (mahamudra).
2Shingon Buddhism, found in Japan, includes many esoteric practices which are similar to those used in Tibetan Vajrayana Buddhism.
Vajrayana Buddhism claims to provide an accelerated path to enlightenment. This is achieved through use of tantra techniques, which are practical aids to spiritual development, and esoteric transmission (explained below). Whereas earlier schools might provide ways to achieve nirvana over the course of many lifetimes, Vajrayana techniques are said to make full enlightenment or Buddhahood possible in a shorter time, perhaps in a single lifetime. Vajrayana Buddhists do not claim that Theravada or Mahayana practices are invalid, only that they represent slower paths. It should also be noted that the goal of the Mahayana and Vajrayana is the attainment of Buddhahood, whereas the goal for Theravada practice is liberation from the cycle of rebirth (samsara) by achieving Nirvana.
Vajrayana partially relies on various tantric techniques rooted in scriptures known as tantras, written in India. The most important aspect of the tantric path is to 'use the result as the Path'; which means that rather than placing full enlightenment as a goal far away in the future, one tries to identify with the enlightened body, speech and mind of a Buddha. The buddha-form which one can best relate to is called the yidam (Tibetan), ishtadevata (Sanskrit) or 'personal buddha-form'. In order to achieve this self-identification with a buddha-form, much symbolism and visualization is used in Buddhist tantric techniques.
Secrecy is a cornerstone of tantric Buddhism, simply to avoid the practices from harming oneself and others without proper guidance. It is not even allowed to explain the full symbolism and psychology of the practice to the uninitiated, which leads to misunderstanding and dismissal. Tantric techniques may initially appear to consist of ritualistic nonsense; however, it should only be practiced on the basis of a thorough understanding of Buddhist philosophy and strictly following the traditions.
While tantra and esoterism distinguish Vajrayana Buddhism, it is, from the Tibetan Buddhist point of view, nonetheless primarily a form of Mahayana Buddhism. Sutras important to Mahayana are generally important to Vajrayana, although Vajrayana adds some of its own (see Buddhist texts, list of sutras, Tibetan Buddhist canon). The importance of bodhisattvas and a pantheon of deities in Mahayana carries over to Vajrayana, as well as the perspective that Buddhism and Buddhist spiritual practice are not intended just for ordained monks, but for the laity too.
The Japanese Vajrayana teacher Kukai expressed a view contrary to this by making a clear distinction between Mahayana and Vajrayana. Kukai characterises the Mahayana in its entirety as exoteric, and therefore provisional. From this point of view the esoteric Vajrayana is the only Buddhist teaching which is not a compromise with the limited nature of the audience to which it is directed, since the teachings are said to be the Dharmakaya (the principle of enlightenment) in the form of Mahavairocana, engaging in a monologue with himself. From this view the Hinayana and Mahayana are provisional and compromised aspects of the Vajrayana - rather than seeing the Vajrayana as primarily a form of Mahayana Buddhism.
Some aspects of Vajrayana have also filtered back into Mahayana. In particular, the Vajrayana fondness for powerful symbols may be found in weakened form in Mahayana temples where protector deities may be found glaring down at visitors.