Classical yoga is based upon ashtanga yoga as compiled by the sage Patanjali in the Yoga Sutras.
Ashtanga yoga means the eight limbs of yoga practice which refer to yama, niyama, asana, pranayama, pratyahara, dharana, dhyana, samadhi.
Through the eight limbs of yoga, the practice can be modified to suit every practitioner level.
Yoga is the cessation of the fluctuations of the mind. - Maharishi Patanjali