Mostly people are doing the asanas [physical postures] in the mistaken belief this is yoga. In fact the physical side of the practice is only there to support the effort required to meditate.
The source text is the 'Yoga Sutras of Patanjali'.
It explains there are 3 stages to meditation:-
Dharana ... withdrawing attention away from sensory stimulation and instead concentrating the mind upon a single task [or object]
Dhyana ... maintaining unbroken attention on that chosen object [so called 'one-pointedness' of mind]
Samadhi ... a trance state in which that chosen object of concentration is removed and the result is a mind completely devoid of content [a state in which no conscious experience is registered in the mind]
