Hatha yoga, pronounced "ha-tuh", is also known as hatha vidya or the "science of hatha" yoga; this word comes from combining the two Sanskrit terms "hat" meaning sun and "ha" meaning moon. The word "hat" refers to the solar nadi (pingala) in the subtle body and "ha" the lunar channel (ida). Hatha yoga is what most people associate with the word "yoga." It is part of the Hindu traditions of Yoga and Tantra, and is seen by many as a path leading to the ultimate goal of Raja Yoga, or contemplation of the One Reality.
Hatha yoga primarily concerns itself with asanas or postures. Asanas are contemplative in nature and were originally intuited by yogis during meditation; the Kundalini naturally brings forth these postures or movements, called Kriyas, during deep meditation. These movements help to remove blockages (disease) in the causal, subtle, and physical bodies. In the West, outside of Hindu culture, "Yoga" is usually understood to refer to "hatha yoga." Hatha Yoga is, however, a particular system propagated by Swami Swatamarama, a yogic sage of the 15th century in India.
Hatha represents opposing energies: hot and cold, male and female, positive and negative, similar but not completely analogous to yin and yang. Hatha yoga attempts to balance mind and body via physical exercises, or "asanas", controlled breathing, and the calming of the mind through relaxation and meditation. Asanas teach teach poise, balance & strength and were originally (and still) practiced to improve the body's physical health and clear the mind in preparation for meditation in the pursuit of enlightenment.
The Hatha Yoga Pradipika
The most fundamental text of Hatha Yoga is the Hatha Yoga Pradipika, written by Swami Swatamarama, a disciple of Swami Goraknath. In great detail it lists all the main asanas, pranayama, mudra and bandha that are familiar to today's yoga student. It runs in the line of Hindu Yoga (to distinguish from Buddhist and Jain yoga) and is dedicated to Lord Adinath, a name for Lord Shiva (the Hindu god of destruction), who is alleged to have imparted the secret of Hatha Yoga to his divine consort Parvati.
It is common for yogins and tantriks of several disciplines to dedicate their practices to a deity under the Hindu ishta-devata concept (see Patanjali's Yoga Sutras) while always striving to achieve beyond that: Brahman. Hindu philosophy in the Vedanta and Yoga streams, as the reader will remember, views only one thing as being ultimately real: Satchidananda Atman, the Existence-Consciousness-Blissful Self. Very Upanishadic in its notions, worship of Gods is a secondary means of focus on the higher being, a conduit to realization of the Divine Ground. Hatha Yoga follows in that vein and thus successfully transcends being particularly grounded in any one religion.
By balancing two streams, often known as ida (mental) and pingala (bodily) currents, the sushumna nadi (current of the Self) is said to rise, opening various chakras (cosmic powerpoints within the body, starting from the base of the spine and ending right above the head) until samadhi is attained.
It is through the forging a powerful depth of concentration and mastery of the body and mind, Hatha Yoga practices seek to still the mental waters and allow for apprehension of oneself as that which one always was, Brahman. Hatha Yoga is essentially a manual for scientifically taking one's body through stages of control to a point at which one-pointed focus on the unmanifested brahman is possible: it is said to take its practitioner to the peaks of Raja Yoga.
In The West, hatha yoga has become wildly popular as a purely physical exercise regimen divorced of its original purpose. Currently, it is estimated that about 30 million Americans practice hatha yoga. But it is still followed in a manner consistent with tradition throughout the Indian subcontinent. The traditional guru-student relationship that exists without sanction from organized institutions, and which gave rise to all the great yogins who made way into international consciousness in the 20th century, has been maintained in Indian, Nepalese and some Tibetan circles.