Asana, literally means 'seat'. There are some descriptions in early yoga texts of how a seat should be for a yoga practitioner during meditation - raised, regular, even surface, clean and so on.
At some point asana acquired a new meaning as 'the seated posture' used by yogis during their meditative practice. The Yogayajnavalkya Samhita describes some of the more commonly used seated postures. These seated postures evolved as the best means of allowing long periods of sitting in a way which supported and facilitated the corrrect depth of inner focus and the appropriate positioning of the five pranas in order to bring about the inward changes that lead towards Samadhi (a deep meditative state that will be described in its own section).
At a later date, seemingly much later, asana came to mean all the physical postures adopted to assist the yogi in his pursuit of acquiing a 'diamond body' that could support the work and the heat nedded to bring about enlightenment. Certainly there is a more detailed description in the Hatha Yoga Pradipika (one of the more important, remaining hatha yoga texts) than there is in the Yoga Sutras where Patanjali defines Astanga Yoga and its components. Patanjali doesn't have much to say about Asana at all - other than that as a posture it should have the qualities of steadiness and comfort (sukha). It's worth noting that this latter word chosen by Patanjali - sukha - also has overtones of sweetness and pleasure. Think Sucre - which is the french for sugar and if you happen to have a sweet tooth, you'll know exactly what Patanjali was getting at.
In modern yoga there are hundreds of postures, some so demanding it's almost impossible to see how they can be achieved, never mind with the qualities of steadiness and comfort. In part this is because of the revamping of yoga by Krishamacharya in the early 1920s, in order to make it relevant and popular in India once more. If you want to know more about this story there is a very good book by Mark Singleton, called 'Yoga Body'.
The new posture work turned out to be very popular, so much so that it now dominates yoga to the point where we could justifiably call Astanga yoga - Eka-anga yoga - one limbed yoga and you don't need to dwell on that image for too long without seeing how that might be less than useful! A really good question to ask yourself is this: 'Do I want to become a master of yoga postures or do I want to develop all the aspects of my being?' Another good question is 'what is this posture for?'
But, it is the way it is and, becasue of Iyengar, Pattabhi Jois and others taught by Krishanamacharya, millions of people around the world treasure yoga as an important part of their life.
In the 1970s Krishamacharya's son, Desikachar, turned his attention to the problem of how to get the other seven limbs working again. It's largely through him that we see an emergence of 'pranayma breathing in posture', both when working dynamically and during stay. We also see a slightly different take on the Yamas and Niyamas so that they can be worked on and developed during a posture-centred practice. The same can be said of Pratyhara and dharana.
In his current work, Paul Harvey has begun a partial synthesis of the traditional with the modern. drawing on both his years of study with Desikachar and his own growing insight to combine not only the traditional aspects of Astanga Yoga with the more modern approach that has its roots in Hatha yoga. It's intereting, absorbing and thought-provoking work.
There is more about Asana in yoga practice - but on another page, headed, unsurprisingly, 'Asana Practice'.