Yoga is one of the six Indian philosophical schools collectively known as darsana. The other five are nyaya, vaisesika, samkhya, mimamsa and Vedanta.
Darsana means ‘sight’ or ‘view’.
Yoga was first recognised in the Vedas, the oldest ‘sacred knowledge or wisdom’ of Indian philosophies and practises.
Patanjali Maharshi systemized Yoga as a special darsana around 200 BC. Today the Sutras are seen as the most authoritative work on Yoga philosophy, psychology and practice.
“The most important text as far as my father was concerned was always Patanjali’s Yoga Sutra. The other texts were certainly useful, but there was no doubt in his mind concerning the relevance of the Yoga Sutra”. Desikachar
The word Yoga comes from the Sanskrit root ‘yuj’ meaning to ‘unite’, ‘yoke’, harness’ or to direct one’s attention on.
Patanjali explains in Sutra 1:2 that the goal of Yoga is “Yogas Citta Vrtti Nirodhah” which translates as,
“Yoga is the ability to direct the mind exclusively toward an object and sustain that direction without any distractions.”
Yoga Sutras of Patanjali 
He explains that this is the goal of Yoga and for a keen student this one would be enough: the rest of the Sutras only elaborate on this one.
“Yoga is the method by which the restless mind is calmed and the energy directed into constructive channels.”
B.K.S Iyengar 
Our natural state is to be present and at peace.
We are often stuck in systems of thought, emotions (stress), habitual behaviour, the future (how it should be) or the past (how it should have been).
We are not independent in this state. Our minds are not conscious and in the present.
The Yoga Sutras say, “Yoga is a journey from dependence to independence.” Yoga Sutras of Patanjali
A fundamental teaching of Yoga is the development and alteration of one’s self-awareness and one’s relationship to the world.
In ‘Yoga and Psychotherapy, The Evolution of Consciousness’ the writers say:
“Much of the discipline of Yoga and the process of meditation is aimed at transforming awareness through a gradual but persistent re-shaping habits of attention.”
Yoga is about undoing much of the doing.
“….yoga and meditation do not bring us health and well-being. Rather, they help us to identify and change behaviors, diet, and perceptions that disturb our inner peace, joy, and well-being, thereby allowing our bodies’ exquisite healing mechanisms to work optimally.” Khalsa et al
Eight Limbs of Yoga
Patanjali outlines the eight limbs of yoga:
Yamas (Ethical Disciplines); Niyamas (Self-observation); Asana (Posture); Pranayama (Breath Control); Pratyahara (Sense Withdrawal); Dharana (Concentration on one object); Dhyana (Meditation, steady concentration on one object) and Samadhi (Blissful awareness, a perfect flow of attention on something that provides a super-conscious experience).
Yama is divided into five moral code: Ahimsa (non-violence); Satya (truth); Asteya (non-stealing); Brahmacharya (moderation); Aparigraha (non-possessiveness).
Niyama is also divided into five codes: Saucha (purity, cleanliness); Santosha (contentment); Tapas (practice causing change/ heat); Svadhyaya (study of the self); Isvara Pranidhana (surrender to a higher force/ let go).