Definition of Yoga
Patanjali defines Yoga as “yogash chitta vritti nirodhah”  ||Sutra 2||

This aphorism has been translated in several ways as follows:
•    Yoga is restraining the mind-stuff (Chitta) from taking various forms (Vrittis).
•    Yoga is the control (nirodhah i.e. regulation, channeling, mastery, integration,coordination, stilling, quieting,  setting aside) of the modifications (gross and     subtle thought patterns) of the mind field.
•    Yoga is the restraint of the modifications of the mind.
•    Yoga or Concentration is the hindering of the modifications of the thinking principle.
•    Union (Yoga) is restraining the thought-streams natural to the mind.

To put it simply, the term Yoga means controlling the thought waves in the mind. It is the prevention of the mind from its wanderings, to use common man’s language.

The above definition of Yoga projects three key words Viz.
1, Chitta (mind-stuff)
2. vritti (modifications) and
3. nirodhah (control).

Upon this Aphorism the whole philosophy of the Yoga Darshana (one of the six systems of philosophy) hinges. Hence we have to be clear about what it means.

1. chitta  (Mind-stuff)
Actually mind-stuff means a treasure house of memories; but for our practical purposes we may  take it to mean the mind. Now let us analyze how the mind functions.

According to Patanjali the mind (chitta) is made up of three components, manas, buddhi and ahamkara. Manas is the recording faculty which receives impressions gathered by the sense organs from the outside world. Buddhi is the discriminative faculty which classifies and analyses these impressions and reacts to them. Ahamkara is the ego sense which claims these impressions for its own and stores them up as individual knowledge. All these as a group are also called Antahkarana.

For example, manas reports ‘a large animate object is quickly approaching’. Buddhi decides ‘that it is a bull. It is violent in mood. It is ready to attack someone’. Ahamkara concludes that it is going to ‘attack me.’ It is I who sees the bull. It is I whom am frightened. It is I who is preparing to run away from the scene. When I take shelter above a tree to protect myself Ahamkara adds that this bull which is dangerous is not I and so I should be careful in future whenever I see a bull. Then this mixture of action and reaction is presented to the Purusha, the real Soul, who perceives an object in this mixture

God (Brahman or Paramatman) is omnipresent and exists in all. God in a sentient being is known as Atman or Purusha, the term used by Patanjali. The mind seems to be intelligent and conscious. But Yoga philosophy teaches that it is not. It has only a borrowed intelligence. The Atman is intelligence itself; pure consciousness. The mind merely reflects that consciousness and so appears to be conscious.

For example many times it so happens that when a marriage procession with a very loud sound of crackers and band music moves along the road in front of your house and yet you do not hear any thing at all because your mind has not attached itself to the organ of hearing i.e. your mind is not in a state of consciousness. You will say that you were absent-minded at that time. As soon as somebody taps you on the back you immediately experience the horrible noise of the procession indicating that your mind has come back to the state of consciousness.  Thus in reality the mind is not sentient; yet it appears to be conscious because it is supported by the intelligent Soul is behind it.

Knowledge or perception is a thought wave (Vritti) in the mind. All knowledge is therefore objective. According to Patanjali even intuitional knowledge or self knowledge or introspectional knowledge is objective knowledge since the mind is not the real seer or the observer, but only an instrument of knowledge, an object of perception like the external world. The atman is the real seer who remains unknown.

When we see an object we say we know this. This awareness that we know is caused by ahmakara or ego sense and not by the Atman. This kind of ego sense is caused by the identification of the Atman with the mind, senses etc. It is just like a miniature electric bulb saying that ‘I am the electric current’ when the light is switched on. Such identification or super-imposition of one over the other is as absurd as the ego’s claim to be the Atman. But we should always remember that electric current is present in the lighted bulb and the atman is in all things, everywhere.

2. vritti (modifications)
Vrittis are the waves and ripples arising in the mind when external causes impinge on it. These Vrittis are our mental responses to the stimuli we receive from the universe around us with all its myriad attractions, distractions, pleasure, pain, joy, sorrow etc. The ego sense identifies itself with these waves. If the ripples of thought are pleasant we say we are happy i.e. the ego sense feels ‘I am happy’. If the thought waves are not pleasant the same ego sense says ‘I am unhappy’. This false identification of the ego sense with the I (or the indweller, Atman) is the cause of all our miseries.

The Atman remains beyond the power of the thought waves. It is eternally pure, enlightened, and free - the only true, unchanging happiness (satyam, shivam, sundaram).It follows, therefore, that man can never know his real Self as long as the thought waves and the ego sense are being identified with one another or mixed up with each other. In order to become enlightened we must bring the thought waves under control so that this false identification of the I with the ego sense will cease altogether.

The action of the thought waves can be described by means of an example. The bottom of a lake we cannot see, because its surface is covered with ripples. It is only possible for us to catch a glimpse of the bottom, when the ripples have subsided, and the water is calm. If the water is muddy or is agitated all the time, the bottom will not be seen. If it is clear, and there are no waves, we shall see the bottom. The bottom of the lake is our own true Self (Atman); the lake is the mind (Chitta) and the waves represent the wanderings of the mind (Vrittis).

Calm man (sattwic person) is the one who has control over the mind waves. Activity is the manifestation of inferior strength, while calmness is of the superior.

The mind is always trying to get back to its natural pure state, but the organs draw it out. To restrain it, to check this outward tendency, and to start it on the return journey to the essence of intelligence is the first step in Yoga, because only in this way can the mind gets into its proper course.

As soon as the waves have stopped, and the lake has become quiet, we see its bottom. So with the mind; when it is calm, we see what our own nature is; we do not mix up ourselves but remain our own selves. At all other times (other than that of concentration) the man is identified with the modifications. For instance, someone blames me; this produces a modification, Vritti, in my mind, and I identify myself with it, and the result is misery. If my mind is not affected or modified despite the accusation hurled at me, nothing happens to me, I am the least agitated, I am a sthita prajna in Gita’s language.

3.  nirodhah (control)
We have so far examined the structure of the mind and its itinerant nature. In conclusion Patanjali advocated the necessity to control its peripatetic tendencies. When we say we have to control the modifications of the mind, immediately two questions arise. They are why we should control the mind and how we should go about to control the mind.

The answer to the first question is that control of mind is necessary for knowing ourselves, who we really are. As per Advaita philosophy we are nothing but the eternal Brahman or Paramatman and because of our ignorance we do not realize that fact. Hence the goal of life is Self-Realisation or Moksha or Kaivalya. To put it simply, the aim of life is to free ourselves from the cycle of births and deaths. While Yoga is the vehicle for traveling to that destination, the process of the control of thought waves in the mind is the fuel to that vehicle.

Control of thought waves does not refer to momentary or superficial control. It implies that we have to do much more difficult thing which is to unlearn the false identification of the thought waves with the ego sense. The process of unlearning involves a complete transformation of the character of the mind or dehypnotisation of the mind as Swami Vivekananda calls it.

Patanjali says that many types of minds are not ready to absorb the unlearning practices. They are scattered minds, flabby minds, restless minds, passionate minds which are unable to concentrate on constructive thought. But every mind, irrespective of its present nature, can ultimately be disciplined and transformed to become one-pointed in Patanjali’s language.

When the lake of the mind becomes still and clear, man knows himself as he really is, always was and always will be. He realizes that he is the Atman. His mistaken belief in himself as a separate, unique individual disappears. His physical body with a name and form is only a cover, a sheath, which he can throw out as he chooses. Such a man is known as a free, illumined soul.

The  Yoga system of  Patanjali lays down elaborate prescriptions for gradually gaining physical and mental control and mastery over the "personal self", both body and mind, until one's consciousness has intensified sufficiently enough to allow for the awareness of one's "real Self" (the soul, or Atman), as distinct from one's feelings, thoughts and actions.

Conclusion
It, therefore, prescribes several sadhanas or directions which are called ashtanga yoga or eight steps of yoga to realize one’s essential nature and get instantly free from samsara, the cycle of transmigration, and attain kaivalya or moksha or nirvana. This realization of the Atman is nothing other than attaining the infinite Brahman.

Patanjali Yoga Sutras mainly deal with mind control to achieve concentration and physical asanas are one of the eight steps or one of the eight limbs of the Yoga sytem (ashtanga yoga) to achieve that purpose of  mind control or chitta vritti nirodhah. Thus the emphasis of Patanjali is more on mind than on physical positions.

Ref:
1.    Complete Works of Swami Vivekananda – Vol I
2.    Patanjali Yoga Sutras By Swami Prabhavananda And Christopher Isherwood