Tantra is often thought of in the West as an exotic way to improve one's sex life. This is a tragic oversimplification of a rich and honorable spiritual tradition.
Tantra is a relatively modern development within Yoga and Hindu traditions, having emerged in the Middle Ages in response to what had become excessively patriarchal and world-denying practices in India.
The Tantric masters turned their devotions to the various Goddesses of the Hindu pantheon (Kali, Durga, Lakshmi, etc.). In the spiritual language of Hinduism, when God is conceived of as a male/female duality, the male divinity is pure spirit, inert, while the female aspect is the divine radiance or power (Shakti) through which creation manifests. In other words, all that is perceived or experienced as part of the manifest world is Shakti, the Goddess. One who worships the Goddess, the emanating power of the Divine, is a Shakta.
The Tantric tradition asserts that all of creation, not simply the transcendental, is divine, for everything equally originates from God. Everything can, therefore, be a path back to the divine so long as one does not become attached to the changing world of phenomenon.
As to the sexual aspect of Tantra, it is only practiced by certain schools of Tantra, and even then with a deep philosophical underpinning. Recognizing that sexual passion is one of the strongest forces driving individuals, some Tantric masters reasoned that its power could be harnessed to turn the mind toward God rather than keeping the individual trapped in materiality. Tantric sexual practices involve seeing the Divine manifest in one's mate, sublimating the sensual aspects of sex toward developing internalized spiritual energies, turning sexual union into a loving and energetically charged form of worship and meditation.
Tantrism is particularly concerned with precise awareness of the pathway of subtle energies within the body during meditation and other spiritual practices. What is not widely known is that the notion of the chakra system, with its seven primary energetic centers along the cerebro-spinal axis, comes to us through the tradition of Tantra.
Hatha Yoga, with its elaborate physical exercises, is considered by many scholars to actually be a branch of Tantra. There has also been a very close link between Tantrism and Tibetan Buddhism.