While Yama and Niyama indicate things one should bstain from and ones to follow, Asana speaks about postures Prathyahaara, Dhyana, Dharana explains the developments int the art of concentration.
These three terms, Prathyaahara, Dhyana and Dharana are different stages and many are not aware of the subtle, essential differences between them.
Prathyaahara is the withdrawal of senses and the focusing of the mind on the object.
Here one is aware that, apart from the object to be focussed upon, of the objects other than the object being concentrated upon.
Dhayana is that stage where the object and the one who is concentrating the object are present in the mind of the one who is concentrating.
In Dharana this duality of the Perceiver and the Perceived disappears.
This is the culminating step of Yoga.
