The breath is termed as Prana and Pranayama literally means controlling the breath. Basically, Prana is the energy which is responsible for life and yama is control and hence Pranayama is considered as the energy that controls the life force. This Pranic energy can be controlled in order to achieve a stable mind and body.
The five types of Prana that are responsible for various activities of the body are Prana, Vyan, Apana, Samana and Udana, out of which Prana and Apana plays a vital role. Prana is the flow of energy in upward direction and Apana is the flow of energy in the downward direction. Through practice of Pranayama a balance of these energies can be achieved.
When the breath that flows in, is neutralized by the breath that flows out a perfect balance and relaxation of the body activities are realized. Pranayama, which is better known as the breathing technique goes in line with the pose or Asana. In the Ashtanga yoga sutras, the pranayama practice is considered as the best form of discipline for the body and mind. These practices produce the physical heat called tapas, which purifies the nerves and improves physical and mental well-being.
Ashtanga Yoga has Pranayama as its fourth limb. This is because, without thorough understanding of the Yama and Niyama and without practicing the Asanas, it is difficult to practice Pranayama. Pranayama requires a certain amount of preparation prior to practice. As it is concerned with breathing, it may do more harm than good when done wrongly without proper guidance. Hence it should be practiced only under expert guidance.