The Yoga Of Bodily Mastery
The word Hatha is derived from two sanskrit words - Ha ("sun") and Tha ("moon"). The sun and moon represent the polar energies of both the physical and subtle/psychic bodies. The purpose of Hatha Yoga is to return these oppositions (male and femal, positive and negative etc) to a state of balance, thereby promoting physical, mental and spiritual harmony.
Body, mind and spirit are cleansed and purified through the practices of Asana, Pranayama and Shat Kriya.
Asana - the practice of physical postures which promote suppleness, agility, tranquility, inner calm and well-being. Asanas act on the physical body by stimulating muscles, nerves, organs and glands. The asanas develop control of the body and prepare it for the practice of meditation. Mental stamina and concentration are developed by "holding" the asanas in conjunction with deep breathing techniques.
Pranayama - breathing practices which regulate the flow of prana (life force/vital energy within the body. Pranayama brings stillness and tranquility to the mind, and is traditionally practiced prior to meditation. The deep breathing required for pranayama improves lung function (encouraging the full use of the lungs), reduces stress levels and cleanses the lungs of toxins.
Shat Kriya - the "six actions", a system of cleansing and purification for the physical body. Toxins are removed from all the systems of the body, the mind becomes clearer and well-being restored on every level. The six cleansing practices are: neti (nasal cleansing), dhauti (cleansing of the digestive tract), nauli (tones and strengthens abdominal organs), basti (cleansing of the colon/large intestine), kapalabhati (breathing techniques that increase oxygen flow and "cleanse" the brain) and trataka (steady-eye gazing - improves concentration and focus, and is also used as a meditative technique).
Hatha Yoga is integral to the practice of Raj Yoga as described by Patanjali in the Yoga Sutras. The practices of Hatha Yoga systematically cleanse and refine the body/mind/soul and make it "fit" for the practice of meditation, and the attainment of Samadhi (Enlightenment/Supreme Bliss).
Hatha Yoga is the system of Yoga most commonly practised in the West. In that context it is often seen as a physical discipline, without reference to its spiritual foundations. The benefits to physical and mental health can be achieved without reference to the spiritual goals. However, over a period of time the more subtle effects of Hatha Yoga on spiritual awareness are felt by most practitioners. As they look back over a practice that was commenced for purely physical reasons they are often astounded at the profound changes in other aspects of their lives also.
"Hatha Yoga works upon the body, purifying it and perfecting it, and through the body upon the mind"
James Hewitt - The Complete Yoga Book