Patanjali’s Yoga Sutras form the foundational basis of classical Yoga philosophy. It appears that Yoga Sutras were influenced by the Upanishad, the Samkhya system and the Mahabharata (of which the Bhagavad Gita is a part) and possibly also by Jain and Buddhist thought during the time the Sutras are considered to have been written. Patanjali’s Yoga Sutras seem to be mostly concerned about the state of the mind and perhaps, even morality. Later evolution of Yoga was more tantric in nature and focused on the divinity of the body in pursuit of transcendence of the world. Despite the fact that modern day Yoga practitioners view Yoga as primarily being a body centric practice, Patanjali Yoga Sutras has still seen a revival in the last couple of centuries and has gained in prominence.
Yoga is one of six darshanas that are recognized as orthodox Hindu philosophical systems because they accept the Vedic revelation as authoritative. The other five are – Vedanta, Mimamsa, Vaisesika, Nyaya, and Samkhya. Each of these systems has a defining text written in the sutra style of pithy aphorisms, that generally requires a commentary or verbal explanation. In in the context of Yoga Darshana, Patañjali’s Yoga Sutras is held in high regard. This is true even today, when Yoga has evolved (and some would say – diverted) considerably from the teachings of Patañjali.
The precise dating of Yoga Sutras has not been possible. Interestingly, this appears to be due to lack of agreement on the correct identity of the author. A grammarian named Patañjali lived in the 2nd century BC, but the Sutras appear to reference Bhagavad Gita, so modern scholars suggest another date and the author probably being another Patañjali who lived in the 2nd or 3rd centuries AD.
Contrary to popular belief (and the way the meaning of ‘Sutras’ is generally understood today) – Patañjali’s Yoga Sutras is not exactly a reference step-by-step Yoga practice manual. Instead, it limits itself to outlining the general philosophy of Yoga and its benefits. This has allowed different types of Yoga practices that would take shape later to still cite the Yoga Sutras as their original source.
YOGA SUTRAS – THE PREMISE
Even though, modern day Yoga practitioners consider the philosophical aspects of Yoga to be advaitic in general, the Yoga Sutras seem to be more aligned with the Samkhya view of the world and the concept of the individual self (purusha). Samkhya seeks to bring out this true self that it considers to be concealed due to our false association with matter (world of sense objects) resulting from ignorance (lack of knowledge or avidya). The mental impressions that we gather (due to avidya) become attached to the true self (purusha) and carries it forward as it transmigrates from body to body. The Yoga Sutras seek to cease this suspension of the external mental focus, so that the impressions we gather do not to attach themselves to the true self (purusha) and the veil of avidya can be lifted through intense transformative practices.
The practices that the Yoga Sutras recommend (though it does not lay down step–by-step manual like guidelines), are meant to still the mind such that the creation of new impressions is prevented which in turn will then cease to generate further rebirths. This emphasis on moksha (liberation from rebirths) bears similarities with the Samkhya and Vedanta systems, with the Jain and Buddhist traditions, and with most of the devotional expressions of Hinduism as well.
CONTENT OF YOGA SUTRAS
Patanjali’s Yoga Sutras comprise of the following FOUR padas – Samadhi-pada, Sadhana-pada, Vibhuti-pada and the Kaivalya-pada – containing in all 196 aphorisms:
The Samadhi-pada which is the first section of the Yoga Sutras explains the type of Yoga Patanjali is advocating and what Yoga will do for its practitioners.
In the following first four lines of this pada, the definition of Yoga is already firmly established:
Yogash chitta vritti nirodhah
Tada drashtuh svarupe avasthanam
vritti sarupyam itaratra
[ Here is the teaching on Yoga. Yoga is the restriction of the movements of the mind. When this is achieved, the witness comes Otherwise the witness assumes the identity dictated by the movement of the mind.]
Yogash chitta vritti nirodhah (Yoga is the restriction of the movements of the mind) has become a famous modern-day definitional verse for Yoga. At the same time, this verse also establishes that Patanjali’s yoga is about the mind rather than of the body (the focus of contemporary Yoga). The following sutras of this pada seek to expand on the idea of chitta-vritti and how Nirodha (cessation of the chitta-vrittis) can be achieved. It focuses primarily on abhyasa (practice) and vairagya (dispassion or detachment). These are similar practices to those suggested by Krishna in Bhagavad Gita.
In emphasizing the necessity of endeavour (tivra -samveganam asannah), Patanjali explains that the nirodha state is very near for those who display ardent intensity in their practice. Interestingly, Patanjali also talks about devotion to the Lord (Isvara-pranidhanad va) in the path of Yoga. To some scholars, this suggests an awareness of the teachings of Bhagavad-Gita.
The second chapter (Sadhana-pada) deals with practical steps by which stilling of the mind may be achieved and introduces the famous eight limbs of Yoga (ashtanga) . The term sadhana is still commonly employed in contemporary Hinduism to indicate the regulated practices or rituals (especially those that involve intense determination) that are undertook in pursuit of goals. Before getting into the explanation of sadhana,Patanjali makes two preliminary points. The sutras 1 to 17, talk about the problem of human existence (almost in parallel to Buddhist belief)-that life is rife with affliction and suffering that arise out of karmic reactions. Then in the second section (sutras 18 to 27), it is mentioned that the problem can be addressed and suggests that our existence is solely for the purpose of enabling the soul to liberate itself from affliction. The following eight limbs of Yoga are outlined with most attention being paid to the first two of the limbs, yama and niyama.
Yamas can be thought of as moral imperatives.  The five yamas listed are: Ahimsa (Nonviolence), Satya (truthfulness), Asteya (non-stealing), Brahmacharya (chastity) and Aparigraha (non-possessiveness).
The second limb of Patanjali’s Yoga path is called niyama, which includes virtuous habits, behaviors and observances such as: Sauca (purity, clearness of mind, speech and body), Santosa ( contentment), Tapas (perseverance, austerity), Svadhyaya (study of Vedas) and Isvarapranidhana (contemplation of the Ishvara, God/Supreme Being) 
Asana is a posture that one can hold for a period of time, staying relaxed, steady, comfortable and motionless. Contemporary yoga can most easily relate to his limb of Yoga. However, Patañjali devotes only three sutras to asana and it does not appear that he was aware of the various bodily postures that are commonly seen in modern day hatha-yoga. Patanjali does not list any specific asana, except the terse suggestion, “posture one can hold with comfort and motionlessness”.Āraṇya translates verse II.47 of Yoga sutra as, “asanas are perfected over time by relaxation of effort with meditation on the infinite”; this combination and practice stops the quivering of body  .The posture that causes pain or restlessness is not a yogic posture.
Prāṇāyāma is made out of two Sanskrit words prāṇa (breath) and āyāma (restraining, extending, stretching). After a desired posture (Āsana) has been achieved, Patanjali recommends the next limb of yoga, prāṇāyāma, which is the practice of consciously regulating breath (inhalation and exhalation).This is done in several ways, inhaling and then suspending exhalation for a period, exhaling and then suspending inhalation for a period, slowing the inhalation and exhalation, consciously changing the time/length of breath (deep, short breathing). 
Pratyahara is a process of withdrawing from one’s thoughts from external objects, things, person, and situation. It is turning one’s attention to one’s true Self, one’s inner world, experiencing and examining self. Pratyahara is not consciously closing one’s eyes to the sensory world; it is consciously closing one’s mind processes to the sensory world. Pratyahara empowers one to stop being controlled by the external world, fetch one’s attention to seek self-knowledge and experience the freedom innate in one’s inner world. Pratyahara marks the transition of yoga experience from first four limbs that perfect external forms to last three limbs that perfect inner state, from outside to inside, from outer sphere of body to inner sphere of spirit. 
Dharana means concentration, introspective focus and one-pointedness of mind. Dharana as the sixth limb of yoga, is holding one’s mind onto a particular inner state, subject or topic of one’s mind. The mind (not sensory organ) is fixed on a mantra, or one’s breath/navel/tip of tongue/any place, or an object one wants to observe, or a concept/idea in one’s mind. Fixing the mind means one-pointed focus, without drifting of mind, and without jumping from one topic to another.
Dhyana literally means “contemplation, reflection” and “profound, abstract meditation”. Dhyana is contemplating, reflecting on whatever Dharana has focused on. If in the sixth limb of yoga one focused on a personal deity, Dhyana is its contemplation. If the concentration was on one object, Dhyana is non-judgmental, non-presumptuous observation of that object. If the focus was on a concept/idea, Dhyana is contemplating that concept/idea in all its aspects, forms and consequences. Dhyana is uninterrupted train of thought, current of cognition, flow of awareness. Dhyana is integrally related to Dharana, one leads to other. Dharana is a state of mind, Dhyana the process of mind. Dhyana is distinct from Dharana in that the meditator becomes actively engaged with its focus. Patanjali defines contemplation (Dhyana) as the mind process, where the mind is fixed on something, and then there is “a course of uniform modification of knowledge”.
Samadhi is oneness with the subject of meditation. When this happens, there is no distinction between the actor of meditation, the act of meditation and the subject of meditation. Samadhi is that spiritual state when one’s mind is so absorbed in whatever it is contemplating on, that the mind loses the sense of its own identity. The thinker, the thought process and the thought fuse with the subject of thought. There is only oneness, samadhi. 
Vibhuti is the Sanskrit word for “power” or “manifestation”. Patanjali indicates that Supranormal powers (siddhis) are acquired by the practice of yoga. Combined simultaneous practice of Dharana, Dhyana and Samadhi is referred to as Samyama, and is considered a tool of achieving various Siddhis. The temptation of these powers should be avoided and the attention should be fixed only on liberation. The purpose of using samadhi is not to gain siddhis but to achieve Kaivalya. Siddhis are but distractions from Kaivalaya and are to be discouraged.
Kaivalya literally means “isolation”, but as used in the Sutras stands for emancipation or liberation and is used interchangeably with moksha (liberation), separation of purusha from prakriti so that the soul no longer has to experience the misery and death that arise from this unwanted association. Patañjali considers this as the ultimate goal of the practice and philosophy he is advocating.
The Indian yoga scene before the 20th century is thought to have been dominated by the Bhagavad Gita, Yoga Vasistha, texts attributed to Yajnavalkya and Hiranyagarbha, as well as literature on hatha yoga, tantric yoga and pashupata yoga rather than the Yoga Sūtras of Patañjali. Patanjali’s Yoga Sutras fell into obscurity for nearly 700 years from the 12th to 19th century, and saw a revival in late 19th century due to the efforts of Swami Vivekananda, the Theosophical Society and others.
Modern scholars consider the Yoga Sūtras of Patañjali formulations to be one of the foundations of classical Yoga philosophy of Hinduism. So even while contemporary Yoga has become very body-centric, the Yoga Sutras are still highly regarded to as providing the defining framework for Yoga in general.
(1) Oxford Centre For Hindu Studies – Philosophy of Yoga