Historical Tantra can be very confusing. In the sense of being a document, a tantra is one of many different writings or scriptures. You find tantras in the Vedic Brahmanical tradition, in Buddhism, in the Jaina traditions, in Shaivism and in other traditions as well! S.C. Banerji’s A Companion to Tantra is an exhausting overview of the diversity of Tantric literature.
Tantra can also be viewed as a religious and cultural movement in early Indic culture. Some scholars believe that it may be a continuation of pre-Vedic shamanic traditions. Another view is that it grew as a reaction against the other-worldly focus of the Vedic Brahmanical tradition and of Buddhism. In this sense, Tantra was concerned with this world, using rituals, magic and sex to promote auspiciousness in the everyday lives of people. Similar to rituals all over the world, Tantra was concerned with deities of childbirth, death, disease, good health, curses and good fortune. For an excellent discussion of the origins of yoga and tantra, see Geoffrey Samuel’s work The Origins of Yoga and Tantra: Indic Religions to the Thirteenth Century.
From this perspective, Tantra explored the world rather than seeking the pure atman (supreme consciousness) or seeking liberation from rebirth. Some tantrics rejected Vedic purity so strongly that they created practices involving wild behavior, graveyard rituals, or “impure” (from a Brahmin’s point of view) substances.
Tantra is also a set of highly refined traditions, developed over more than a thousand years, for exploring different states of mind and body. The mantras, yantras, meditations, physical exercises, energetic transmissions and meditations were invented and re-invented by many great teachers. For an overview, see Georg Fuerstein’s Tantra: The Path of Ecstasy. For a clear look at a specific tradition, see Swami Shankaranda’s The Yoga of Kashmir Shaivism. Interestingly enough, threaded through all of these traditions is the notion that ritual is only necessary for those in whom sufficient discrimination is not yet developed.
That brings us to another perspective on Tantra. That it is beyond ritual, beyond deities, beyond anything that is not the universe itself. All of the mental and physical states previously catalogued by humans are made transitory in the ongoing evolution of the universe. This is the perspective explored by Sex and Quantum Physics: How in this modern world can we best explore and connect to the universe?