There is some confusion as to which Patanjali was the author of this book. He has been identified with a grammarian by the same name, but the grammarian's dates do not match the age of the work, as determined by the internal evidence. It's safe to assume that the Sutras were written somewhere between 200 BC and 300 AD.
The techniques described in the book come under the heading of raja yoga, or the royal path to union with the divine. In essence it is mind training more than body training. Patanjali's Yoga Sutras fall under the six darshanas of Hindu or Vedic schools and is a milestone in the history of Yoga along with the Bhagavad Gita and Hatha Yoga Pradipika.
A compilation of Yogic thought that is largely Raja Yogic in nature, it was codified some time between the 2nd century BC and the 3rd century by Patanjali, and prescribes adherence to "eight limbs" (the sum of which constitute "Ashtanga Yoga") to quiet one's mind and merge with the infinite. These eight limbs not only systematized conventional moral principles espoused by the Gita, but elucidated the practice of Raja Yoga in a more detailed manner. Indeed, his "eight-limbed" path has formed the foundation for Raja Yoga and much of Tantra Yoga (a Hindu deific, Shiva-Shakti yoga system) and Vajrayana Buddhism (Buddhist Tantra Yoga) that came after. It goes as follows:
While Patanjali accepts the idea of what he terms "ishwar-devata" (worship of deities as manifestations of the single Brahman), his "ishwar" is not a conventional God and speaks more to a universal Brahman, an impersonal, unknowable, infinite force that is all and transcends all.
The Sanskrit word sutra means "thread" or "aphorism" and for that reason the work is sometimes translated as the Yoga Aphorisms. Patanjali created 4 chapters or books ( = Sanskrit pada), containing in all 195 aphorisms, divided as follows:
The text emphasizes a non-mental way of knowledge as alternative form of knowledge. The procedure to achieve this type of knowledge is schematically outlined in Yoga Sutras, as application of the principle that a steady concentration on a certain object allows identification of the concentrating subject with the object itself and with the process of concentrating. This allows the subject to gain direct knowledge of the object by identification with it, rather than by indirect means, i.e. through mental elaboration of the sense perception of the object. Such process, called samadhi, occurs only when the concentration on the object neutralizes the mental activity, whose waves prevent the individual consciousness to fuse with that of the object.
After the Bhagavad Gita, the next seminal work on Hindu Yoga is Patanjali's "Yoga Sutra."
The Yoga Sutras form the theoretical and philosophical base of all Raja yoga. It can still today be considered the most organized and complete definition of the Raja yoga discipline.