Yoga can be put into 4 categories: Jnana Yoga – the way of wisdom; Bhakti Yoga – the way of emotional rapport; Karma Yoga – the way of dedicated works and Raja Yoga – the way of spiritual practices. What is popularly known as Yoga in the West is called Hatha Yoga and involves physical disciplines including various postures (asanas) which are conducive to healthy life and a strong body.

Living as we are in an age of great stress and turbulence, the regular practice of Yoga can help us enormously in our physical and spiritual development. Physical Yoga, in fact provides a foundation for a deeper enquiry into the working of the human mind, the most intricate mechanism in the cosmos. As refered to in the Vedas: Shariramadyam Shalu Dharma Sadhanam –the body alone is the basis of all other achievements. We must strive to treat the body as our temple.

Yoga in Philosophy:
The ancient seers used Yoga as a means to explore the exterior and the interior world and ultimately achieve the knowledge and wisdom of the Vedas, Upanishads and Shastras which has been passed down to the world.

Classical Indian philosophy is divided into six schools called Shad Darshanas. They are Hyaya, Vaisheshika, Mimamsa, Vedanta, Sankhya and Yoga. These systems or schools are known as "Darshanas" which mean to see or mirrors to see the soul. It may be improper to call them philosophies which are noting but a view of life, whereas the Darshanas are more or less a way of life.

The founders of these systems consolidated and systematised the ideas and practical wisdom which existed much before them, during the various periods and compiled them into Sutras (Aphorisms).
Gautama´s Nyaya Sutra
Kanada´s Vaisheshika Sutras
Kapila´s Sankhya Sutras
Patanjali´s Yoga Sutras
Jaimini´s Purva – Mimamsa Sutras
Badrayan´s Brahma Sutras

Sankhya and Yoga are closely related. Sankhya covers the theoretical part whereas Yoga forms the practical side.

Importance of Yoga:
Literally, Yoga means union. Generally, the word is used for the union of the individual self and the Supreme Self, that is the realization of God. It is to be understood as the integration of KarmaYoga, Gnyana Yoga, Bhakti Yoga and Raja Yoga. So in a wide sense, Yoga signifies spiritual discipline.

Sage Patanjali in his Yoga Sutras defines Yoga as "Chitta Vritti Nirodha" – that which restrains the thought process and makes the mind serene. The fluctuations in the consciousness are restrained. Simplified, it means penetrating to inner levels, from gross to subtle. It signifies the complete course of action up to its ending in spiritual absorption.
According to Sage Patanjali, this is to be achieved by practice eof eight fold path know as Ashtanga Yoga. They are Yama, Niyama, Asana, Pranayama, Pratyahara, Dharana, Dhyan and Samadhi.

The eight limbs of Yoga:
Yamas: Ethical Disciplines – (5)
Ahimsa – Non violence in thought, word or deed.
Satya: Truthfulness in thought, word and deed.
Asteya: Non stealing. Not even the desire to posses what someone else has.
Brahmacharya: Copntinence – behaviour in thought, word and deed which leads to Brahman.
Aparigriha: Not to hoard, non-receiving.

Niyamas: Observances (5)
Saucha: Purification. Internal and External.
Santosha: Contentment.
Tapas: Fervour, burning desire, inner zeal.
Swadhyaya: Study of sciences of the Self.
Ishwara Pranidhan: Surrender to God.

The Yamas and Niyamas form the foundation of Yoga practice. They are to be followed at every moment of one´s life. The advantages of following these are very well enumerated in Yoga Sutras.

Even those who claim not to follow a spiritual path, these ethical observances along with Yoga, build the person´s character and free his mind of doubts by helping him to discriminate between right and wrong. They keep him mentally and physically healthy and along with single pointed attention they take him towards success in any endeavour.
Reference: Dharmavrisingh Mahdia from Yoga Illustrated.