The word yoga means "union" in Sanskrit, the language of ancient India where yoga originated. We can think of yoga as the union occurring between the mind, body and spirit.
Elaborating on yoga can run into volumes. It is more than just a physical treatment of the body. It is a path to spiritual ecstasy. It is a way to reach your identity independent of religion and belief. It is a science, which aims at the harmonious development of the body, the mind and the soul. Yoga aims at controlling the mind and its modifications. It is consists of ancient theories observations and principles about the mind and body connection, which is now being proven by modern Science.
Today mostly, what is known of yoga is Asana (Postures), Pranayam (breathing techniques), Kriya (cleansing activities) Bandha (neuromuscular locks), and Mudra (hand movements).
Originally, Yoga had eight ladders. Yam, Niyam, Asana, Pranayam, Prathyhar, Dharna, Dhyaan, and Samadhi
Yam : This consists of ethical guidelines regarding moral behavior towards others
Niyam : This consists of ethical guidelines regarding moral behavior towards oneself
Asana : Is the practice of yoga Postures
Pranayam : Is the practice of breathing exercises
Prathyahar : Withdrawal of senses from the body, meaning the exterior world is not a distraction to the interior world within oneself.
Dharna : Concentration, meaning the ability to focus on something uninterrupted
Dhyaan : Meditation
Samadhi : Is the highest know how in Yoga. This is a state of bliss
The first four ladders, namely, Yam, Niyam, Asana and Pranayam form what is called 'Bahiranga Yoga'(external yoga) and the other four ladders namely Prathyahaar, Dharna, Dhyaan and Samadhi form 'Antaranga Yoga' (internal yoga). To practice Antaranga Yoga one would need a Master, who has traveled well till Samadhi. Without this, practicing Antaranga Yoga will not be possible.
Patanjali divided yoga postures into three:
Yoga regards the body as a vehicle for the soul on its journey towards perfection. It develops not only the body, but also broadens the mental faculties and spiritual capacities.