Patanjali describes asana in the Yoga Sutras as being steady and comfortable. The Bhagavad Gita presents the quality of asana is being stable and erect. Asana should be practiced with effort but without strain until the posture becomes effortless. Through the effortlessness of the asana, we are able to transcend the physical body and merge the mind into the infinite. As Patanjali says, asana needs to have the dual qualities of firmness and lightness. When these dualities are balanced, effort ceases and one can transcend duality and attain equanimity.
Asana functions to purify and align the body. It also begins the processes of purifying and aligning the nervous system and the mind, which are then firmly established by the practices of pranayama and meditation. Through asana practice the body becomes strong, pure, and flexible.
There are 8,400,000 asanas in the entire yoga system. In the Ashtanga Vinyasa system of yoga selected asanas are strung together in a set sequence to create a series. Each asana is dynamic and effective in its own way. The practice of asana requires the body to be placed into a complex position, often times asking the practitioner to transcend their preconceived limitations. Types of asanas include but are not limited to standing postures, seated postures, forward bends, backward bends, twists, hip openers, inversions, and arm balances.