By Dalai Lama
The Divisions of the Vehicles (excerpt)

Various systems of thought and practice are mentioned in classical Buddhist literature. Such systems are referred to as yanas or "vehicles." There are, for instance, the various vehicles of humans and divine beings in addition to the Buddhist vehicles: the vehicle of individual liberation (hinayana), the vehicle of universal salvation (mahayana), and the vehicle of tantra (vajrayana). In this context, vehicles of humans and divine beings refer to systems that outline the essential training and methods for both fulfilling the major aspirations of this life and, in addition, obtaining a favorable rebirth as either a human or a divine being. Such systems emphasize the importance of maintaining an ethically sound lifestyle--grounded in refraining from engaging in negative actions--since leading a life of righteousness and good behavior is perceived to be the most crucial factor for ensuring a favorable rebirth.
The Buddha also spoke of another category of vehicle, the Brahma Vehicle, comprising principally those techniques of meditation that aim at achieving the highest possible form of life within samsara, the karmically conditioned cycle of existence. Such meditative techniques include, among other things, withdrawing the mind from all external objects, which leads to a state of single-pointedness. The meditative states experienced as a result of having generated single-pointedness of mind are altered states of consciousness that, in terms of their phenomenological aspects and also their mode of engagement with objects, closely correspond to states of existence in the form and formless realms.
From a Buddhist point of view, all these diverse systems are worthy of respect since they all have the potential to bring about great benefit to a large number of sentient beings. However, this does not mean that all these systems are complete in themselves in presenting a path leading to full liberation from suffering and from the cycle of existence. Genuine freedom and liberation can only be achieved when our fundamental ignorance, our habitual misapprehension of the nature of reality, is totally overcome. This ignorance, which underlies all our emotional and cognitive states, is the root factor that binds us to the perpetual cycle of life and death in samsara. The system of thought and practice that presents a complete path towards liberation from this bondage is called the vehicle of the Buddha (buddhayana).
Within the Buddha's Vehicle there are two major systems of thought and practice: the Individual Vehicle, or Hinayana, and the Universal Vehicle, or Mahayana. The former includes the Theravada system, which is the predominant form of Buddhism in many Asian countries, such as Sri Lanka, Thailand, Burma, Cambodia, and others. In classical Buddhist literature, the Individual Vehicle is described as having two main divisions: the Hearers' Vehicle and the Solitary Realizers' Vehicle. A principal difference between the Individual Vehicle and the Universal Vehicle exists in their views on the Buddhist doctrine of selflessness and the scope of its application. The Individual Vehicle expounds the view of selflessness only in relation to person or personal identity but not in relation to things and events in general, whereas in the Universal Vehicle, the principle of selflessness is not confined to the limited scope of the person but encompasses the entire spectrum of existence, all phenomena. In other words, the Universal Vehicle system understands selflessness as a universal principle. Interpreted in this way, the principle of selflessness acquires greater profundity. According to the Universal Vehicle teachings, it is only when a practitioner's experience of selflessness is rooted in this universal interpretation that the experience will bring about the elimination of the delusions and their underlying states of ignorance. It is by eliminating these underlying states of ignorance that we are able to cut off the root of samsara. Furthermore, a profound experience of selflessness can also lead, ultimately, to full enlightenment, a state of total freedom from the subtle imprints and the obstructive habitual tendencies created by our misconception of the nature of reality. The system of thought and practice which presents such a view of selflessness is called Mahayana, the Universal Vehicle.
The Tantric Vehicle, or Vajrayana, which is considered by the Tibetan tradition to be the highest vehicle, is included within the Universal Vehicle. In addition to meditative practices for enhancing one's realization of emptiness and bodhicitta, this system also includes certain advanced techniques for utilizing the various elements of the physical body in one's meditative practice. Such feats are accomplished on the basis of sophisticated yogic practices that principally involve mentally penetrating the essential points within the body where the cakras, or energy centers, are located. By means of this subtle and refined coordination of mind and body, the practitioner is able to accelerate the process of getting at the root of ignorance and completely overcoming its effects and imprints, a process that culminates, finally, in the realization of full enlightenment. This feature--of engaging in meditative practices involving the subtle coordination of both mental and physiological elements within the practitioner--is unique to the Tantric Vehicle.