Pranayama - Control of Breath, Lifeforce EnergyPrana: breath, respiration, wind, lifeforce, energy, vitality, strength
Yama: control (of an aspect of the self), discipline, a means to supreme union
For an introduction to the importance of breathing in yoga, see my earlier post on breath. That post also contains a simple pranayama you can do any time to clear your mind, stop a headache, or improve focus.
Pranayama, breath control, is a powerful means of regulating the mind's machinations and body's processes; it is essential to actually "doing yoga" - yoking / uniting the thoughts with the movement (even stillness) and thus communing with a reality beyond the ego.
Each pranayama practice has a different aim; some balance the energy in the body (such as nadi shodana, for equally stimulating the two sides of the energy body, throughout the length of the spinal chord up into the brain, by activating two major left-right energy channels, or nadis that meet at the nostrils). Pranayama before asana practice may also increase or release heat in the body, and breath control alone can suffice for a physical practice for those who are ailing, recovering, or quite advanced in practice.