About 2000 years ago the Indian sage Patanjali structured and systematized his collected knowledge of yoga in the book: The Yoga Sutras. Patanjali defines the goal of yoga as "the control of mental activities." To achieve this goal we pracitce Ashtanga Yoga which literally means "eight limbs of Yoga".
The four chapters or books (Sanskrit = padas) of the Yoga Sutras consist of a total of 195 aphorisms.
Samadhi refers to a blissful state where the yogi is absorbed into complete self-realization. The chapter describes yoga and then the nature and the means to attain Samadhi.
Sadhana is the Sanskrit word for "practice" or "discipline". This chapter outlines two forms of Yoga: Kriya Yoga (Action Yoga) and Ashtanga Yoga (Eightfold or Eightlimbed Yoga).
Kriya yoga, sometimes called Karma Yoga, is also expounded in Chapter 3 of the Bhagavad Gita. It is the yoga of selfless action and service.
Vibhuti is the Sanskrit word for "power" or "manifestation". Special powers are acquired by the practice of yoga. The temptation of these powers should be avoided and the attention should be fixed only on liberation.
Kaivalya literally means "isolation" or “insignificant”, but as used in the Sutras it stands for emancipation and can be used interchangeably with moksha (= “liberation” as it is used in the Bhagavad Gita). The Kaivalya Pada describes the nature of liberation and the reality of the transcendental self.
Yama and Niyama form the moral and ethic base for the yoga practice. The physical practice of Asanas make the body healthy, strong and flexible. In addition of cleansing the body and a purification of the mind the breathing exercises of Pranayama lift the practitioner to a higher spiritual level.
These first four limbs of Ashtanga Yoga are called outer yoga as they prepare mind and body optimally for the other limps. The limps of the inner yoga – Pratyahara (withdraw of the senses), Dharana (concentration), Dhyana (meditation) and Samadhi (enlightenment) – work on the spiritual and intellectual development of man as they discipline the mind and create harmony between body, mind and soul.