Asana is a Sanskrit word translated into English as a physical yoga position, posture, or pose. There is a difference though between the everyday usage of those words and asana.
Moving into an asana, the body moves consciously with fluid movement which is coordinated with the flow of breath. Asana is conscious movement into a traditional yoga pose. The movement is physically and mentally relaxed. Balanced equilibrium maintains the asana when it is held in a stable state. As the body and breath movements are synchronized in a relaxed way, a full awareness of both is developed. A steady asana is a portal to higher awareness of the body and mind.
Awareness is the heart of meditation and here the meditation object is the physical body or annamaya kosha. There are five koshas or bodies, also known as sheaths, in yoga: annamaya kosha (physical), pranamaya kosha (pure energy or vital life force), manomaya kosha (mental), vigyanamaya kosha (knowledge or wisdom), anandamaya kosha (bliss). The natural path of awareness will flow from body to breath and continue to flow to the increasingly more subtle sheaths.
The traditional hatha asana is a tool to create balance and harmony within body and mind. "Ha" is the moon and "tha" is the sun. In our bodies they represent the mental and the physical, breath and body. A specific asana uses the movement to direct and infuse a particular part of the body with prana or vital energy. This can help relieve tension or tightness, heal, and create a much greater awareness of prana as blockages to its smooth flow are removed. It can help bring that area of the body in harmony with the rest of the body-mind. As the awareness of the way the body moves deepens, bad habits and negative conditioning can be replaced with more conscious, positive movements of the body. Not all asana, or all styles, are suitable for everyone just as all medications are not for everyone.
From the perspective of an asana practice, it is often hard to differentiate between tight muscles (respond to stretching), tense muscles (respond to relaxation), and those that are both. Incorporating breath into the movement helps relieve both. Repetitions help warm the joints and increase the range-of-motion. They may also increase stamina. Holding an asana increases strength and endurance. It can also increase stamina. A hatha practice that incorporates some dynamic asana with some passive ones can often be the most effective.
The most effective asana, in our opinion, is not necessarily the most complex or challenging form, or technique. An effective asana is the most conducive portal to the inner Self or Great Being. It does this by eliminating tension; creating physical, emotional, and mental stability; and establishing a sustainable, steady, balanced space that enables a constant focus or concentration of higher awareness. A simple asana that does this is far more advanced than the most physically challenging one that does not. The traditional hatha asana develops the raja yoga asana which is a sound and stable physical structure so the body does not become a distraction for the higher awareness. This higher awareness is then directed to the inner space with the breath.
With the proliferation of so many styles of hatha yoga, it is natural to be confused about asana styles. Asana is a key component of hatha yoga and in its development in the West, it has taken on so much importance that it is synonymous with all of "yoga." It is important to keep in mind that asana is just one of many aspects of yoga.
Just as a play, music, dance can be interpreted and performed in many ways, so can asana. Think of the body as hardware and the mind as software. Depending on the type of hardware and the software (programming through genes, culture, geography, family, upbringing, etc.), the way each person relates to an asana is different. Depending on the body type, limitations, personality, need, and relationship to a teacher, we gravitate toward a particular expression or style of asana interpretation. Some emphasize alignment, some form and technique, some stamina and endurance, some spirituality, some therapeutic benefits. Each may be a valid aspect and is a reflection of the person who created that style and his or her own hardware and software. There are clear differences in the way various schools and teachers teach: Satyananda Yoga, Integral Yoga, Sivananda Yoga, Himalayan Institute (all raja yogis), and the vastly different styles of Desikachar, Iyengar, Jois, and Bikram Yoga (all hatha yogis). Then add in their students who have developed their own interpretations and brand names! In that respect, it is not much different from dance and music. Our view can and does change over time. Asana, like all of yoga, is transformational. We are all evolving anyway. With yoga it is a conscious and directed change. Our meaning of asana, how we move into and out of it, how we relate to it, what we see through it, is constantly evolving. That, to us, is growth. Choosing what works for your body and personality sensibly is a good choice. Then confusion will not arise.
Some confusion also arises due to the lack of distinction between asana in hatha yoga and asana in raja yoga. This is compounded by the fact that most mass market hatha yoga books refer to Patanjali's Yoga Sutras, which is raja yoga, without understanding how one truly supports the other. There are two types of asanas--the meditation seat asana and the other asanas that are used to make the body strong, flexibile, and disease-free. The Yoga Sutras does not deal with most hatha asana. Traditional hatha texts give a better explanation and purpose of the asanas that are synonymous with yoga. Certainly the Yoga Sutras can and should give the mental perspective, the essential philosphy, and purpose of the practice. Without the foundation of the philosophy, it is not a yoga practice for inner peace. Yoga is not just a "mat practice." The Yoga Sutras delves deep into the mental, wisdom, and bliss sheaths. These are difficult concepts and the body and mind need to be prepared for them through hatha yoga. Hatha texts such as the Hatha Yoga Pradipika and Gherand Samahita are the ones that instruct on asanas. Hatha yoga is the ladder one climbs to reach raja yoga. Hatha yoga's medium is the physical body. Raja yoga is yoga psychology. It's medium is the mind. The two are an inseparable continuum but the mind cannot be understood entirely through the physical body and just asana. Hatha yoga is one segment or limb of a complete therapeutic system that encompasses the total being--body, mind, Spirit.(See the article "What is Yoga?").
There are numerous asanas. Most yoga styles in the West are hatha styles. The hatha asana practice can be passive, dynamic, gymnastic-like, graceful, reverent, or therapeutic, depending on the interpretation. There is initial discomfort and a lot of effort. This is to prepare the body for the raja yoga's sitting asana. The sitting asana looks very easy compared to many hatha asanas, until it has to be held for several hours. In raja yoga, the Yoga Sutras only list passive sitting positions for meditation. Raja yoga asana is a meditation position that gives the body stable rigidity. Discomfort and effort have now disappeared according to the Yoga Sutras. The asana becomes perfect when the effort disappears. There is perfect balance of opposite forces in the body. There is no movement, no motion. This sitting asana is a crucial step in meditation.