Pranayama is generally defined as ‘breath control’. Although this interpretation may seem correct in view of the practices involved, it does not convey the full meaning of the term.  Prana  means ‘vital energy’ or ‘life force’,  ayama  means ‘to expand’, accordingly pranayama is expansion of the vital energy. This vital energy is responsible for all life experiences including physical and spiritual.

Pranayamas harmonise the pranic body which is a network of nerve channels carrying the vital force to each and every cell and organ infusing them with life and dynamism. Pranayamas are also the preparatory practices for the awakening of the chakras  (psychic centres) and aid in the perfection of kriya and kundalini yogas.

In the past pranayamas were taught to a select few as a part of higher yoga practice, but in the course of time Sri Swami Satyananda Saraswati reintroduced the subject of pranayama as a part of daily yoga sadhana. The sequence in which the pranayamas are taught today by many yoga schools were developed by the Bihar School of Yoga.

The main publication dealing with this subject is  Prana and Pranayama

(Ref: www.biharyoga.net)