Pranayama is the fourth anga or organ (branch) of ashtanga yoga as prescribed by Patanjali, the chief guru of Ayurveda. Astanga means eight organ-ed or eight branched). Pranayama is the technique of collecting and storing more prana (life) from the air (oxygen) we breathe, or in simple words pranayama control (ayama) of prana (life force).
Yama, Niyama and Asana are the first three angas (organs or branches).
There are many types of pranayama, eight of them are generally considered to be important. Learning of some of the pranayamas is strictly restricted to learning directly from a teacher (the same way you cant take a postal course for swimming).
Mind and life is essential for the existence of man. If any one of the factors is lost, the person ceases to exist. Prana (life) enters our body while taking shape at womb and leaves us when we die. Prana is omnipresent. It is present in the air we breathe, water we drink and food we eat. You know, the most important of them is air; we cant sustain our prana for even two minutes if we are devoid of all important air.
Several researches suggest the power of pranayama in treating stress related disorders, asthma, and oxidative stress. Yoga practitioners who practice pranayama experience strong and controlled mind.