Asana is one limb of an eight (ashta) limb (anga) path. This is Ashtanga Yoga. Not to be confused the the Ashtanga Yoga of Pattabhi Jois which is a vinyasa style of asana.
The eight limbs are:
Yama - social ethics - five universally accepted moral values
Ahimsa - non violence
Satya - truthfulness
Asteya - non-stealing
Brahmacarya - sexual restraint
Aprigraha - non-hoarding
Niyama - personal values
Saucha - cleanliness
Santosha - contentment
Tapas - austerity or perseverance
Svadhyaya - self-study
Ishvara Pranidhana - surrender to a higher power
asana - body postures
Pranayama - breath regulation
Pratyhara - emancipation of the mind over the senses
Dharana - cultivating conscious awareness
Dhyana - meditation
Samadhi - oneness with the Supreme Being
The first three limbs: Yama, Niyama and Asana, comprise the external level of practice. Observing the tenets of yama and niyama enables the yogi to restrain desires and emotions and maintain amicable relations with all. When asanas are practiced in the right spirit, the yogi ceases to obsess over physical appearance and treats the body with due respect as the vehicle of the soul.
The second two limbs: Pranayama and Pratyhara comprise internal disciplines.
The last three limbs take the practitioner to the very core of their being where Ishvara (god) is said to reside.
Yoga is, after all, not just a physical practice. Yoga darshana is a school of thought that attempts to turn the mind away form desire and kindle in it the spark of spirituality.
In the beginning, the sadhaka (practitioner) proceeds from the outer levels to the inner levels in a linear fashion. However, as the practice progresses, the dividing line between the levels starts to blur. The sadhaka acquires the ability to both access the deeper levels of consciousness through external practices and connect with the physical level through internal practices. Each stage of growth brings fresh enlightenment and new experiences. The individual’s consciousness evolves from gross awareness to subtle perception, until it eventually penetrates the innermost recesses of being.
From this description, you might think that yoga is a discipline meant only for those on a spiritual path. But, this is not true. Yoga is for all those who wish to improve their health and well being. Physical health is as important for those on the spiritual path as it is for the average person. The state of stability, peace and tranquility that is so essential for self-realization is difficult to achieve when unfit. The daily grind fatigues the body, making it stiff and heavy. Asanas stretch the muscles and improve circulation to the affected parts. The organs start functioning optimally and overall health improves. The improved levels of fitness aid the practitioner’s spiritual quest. It is therefore imperative that the layperson as well as the spiritual seeker starts their yoga journey with the study of asana.