Tantra (a Sanskrit word which means "woven together") is a term loosely applied to several divergent and even contradictory schools of Hindu yoga in which the sexual union of male and female is worshipped either in principle or in human practice. It has also come to be applied to sex-based religious practices developed in other religions, including Bon, Tibetan Bhuddism, Taoism, Christianty, Judaism, and Transcendentalism.
Pre-Hindu tantra: Shaktiism and Shaivism
Perhaps the earliest strand of tantra yoga, derived from the Dravidian, pre-Hindu religion of Shaktiism (worship of the goddess in her numerous forms), focuses on yoni puja, a ceremony honouring the vulva -- either of a statue or a living woman. Depending upon the school of study, this puja may involve making offerings of food and liquids while chanting prayers or it may involve the deliberate sexual arousal of a woman who is believed to embody or personify the deity. A related thread of tantra yoga that derives from Shaivism (the worship of the god Shiva, which predates the syncretistic religion now known as Hinduism) has at its center linga puja, a ceremony honouring the penis, often in the form of a natural upright stone.
Contemplative yoga as an influence on tantra
One major strand of Hindu tantra yoga centers on meditation. Allied to non-sexual meditative schools of yoga, such as hatha yoga (the yoga of body posture) and bhakti yoga (the yoga of devotion), this contemplative school of teaching advocates a fairly non-sexual approach to sex religion, in which visualization of a deity, chanting of a mantra, and concentrating on iconographic symbols are the foremost activities. A highly attenuated form of yoni puja is sometimes encountered in this school, with the practitioner meditating upon a yantra or iconographic image -- often a downward-pointing triangle -- that symbolizes the vulva of the goddess.
Kundalini yoga as an influence on tantra
Some Hindu tantra yoga teachers recommend practices that may include meditation but also share elements with kundalini yoga, in which subtle streams of energy of thought to be "raised" in the body by means of conscious posture and strenuous breath control. Most teachers in this school of tantra advocate the retention of semen as a prerequisite for spiritual advancement, although they differ on how much sexual arousal it is good to provoke while retaining semen -- and few have anything at all to say about a possible female counterpart to semen retention as a spiritual path.
Hindu "right hand" and "left hand" path tantra
Best known to Westerners are the several strands of tantra yoga in which worship services take the form of a sexual ritual featuring slow, non-orgasmic intercourse as a prelude to an experience of the divine. This broad category of tantric sex ritualism, which derives from the pre-Hindu religions of Shaktiism and Shaivism, has in turn produced two schools of practice: The "right hand path" is one in which the ritual is more or less seen as a monogamous rite or may be allied to the yoni puja of Shaktiism, while the "left hand path" is one in which dozens -- or hundreds -- of couples may engage in the ritual sex act at the same time, sometimes following the lead of a pair of teachers. This latter path is the one that has earned tantra yoga the reputation of being orgiastic and even "satanic" among thse who are ignorant of its history or prejudiced against sexuality.
Tibetan Buddhism and pre-Buddhist Bon forms of tantra
A modified verion of pre-Hindu tantrism can be found in contemporary Tibetan Buddhism, where it seems to be a blend of pre-Buddhist goddess worship mingled wih influences from the ancient Tibetan animist religion known as Bon. Like Hindu tantra, Tibetan Buddhist tantra encompasses schools of practice that range from the meditational to the sexually active.
Taoist tantric alchemy
That other great Asian religion, Taoism, has its own tantric schools, each with a different view of the role of sexual activity in the life of the aspirant. One strand of Taoist tantra is called tantric alchemy, and, like Western alchemy, it places a certain amount of emphasis on the search for immortality or at least long life. Presumeably influenced by the male-centered, kundalini-derived forms of Hindu tantra, Taoist tantric alchemy involves breath and muscle control and emhasizes the retention of sperm as proof of spiritual attainment. Other Taoist tantra teachers, working out of a paradigm that seems to be derived from Shaktiism, claim that Lao Tzu, the founder of Taoism, was in fact advocating a form of yoni puja or worship of the vulva when he wrote about "the valley spirit."
Judeo-Christian tantra: karezza et al
Going still farther afield, the term tantra is frequently -- for the sake of convenience -- applied to Western religious or spiritual practices in which slow, non-orgasmic sexual union or masturbation creates a path to the experience of spiritual ecstasy. Some of these Western practices arose during the 19th century, apparently by spontaneous discovery -- although one American popularizer of Western sacred sex is known to have travelled to India to study Hindu tantra yoga. Each "discoverer" gave his or her system a unique name -- the Reverend John Humphrey Noyes preached the doctrine of MaleContinence A. E. Newton wrote of The Better Way, Alice Stockham pioneered Karezza, Paschal Beverly Randolph advocated The Anseiratic Mysteries, Thomas Lake Harris practiced a form of breath-eroticism as well as non-corporeal sexual union with beings from other dimensions, Stockham's student John William Lloyd coined the term Magnetation, and Stockham herself published George N. Miller's novel, "Strike of a Sex," in which he described the fictional but Karezza-like "Zugassent's Discovery." While these Western spiritual practices share certain common sexual techniques with traditional Hindu tantra yoga, most of them fit conventiently into Christian, Jewish, or Transcendentalist conceptual frameworks, obviating the need for the practioner to adopt a culturally "foreign" religion.
Sex ritualism: a biological appraoch to the sacred
The sexual rites of tantra yoga and its Western counterparts form the basis for orthodox religious worship services and are at the core of the personal spiritual paths of countless individuals. This has been true for millennia and continues to be the case to this day, despite the persecution of sexuality in most modern civilizations. In my opinion, the reason that such strikingly similar sexual rituals have arisen spontaneously in different eras and places -- and the reason they so easily cross socio-cultural boundaries -- is that sex worship itself is rooted in the neurological hard-wiring of the human body; because it is something which, when practiced correctly, allows the participants to experience what seems to be -- what IS, for all intents and purposes -- the presence of deity in the person of the sex partner.