Not to mention that first guru of Yog[a] is Devadhidev Mahadev Bhole Shankar.
Rahul,
In our vedic land, every vidya(knowledge) has a presiding deity. In line with this tradition, Lord Shiva is the presiding deity and the first guru(teacher) of Yoga. In terms, earthly gurus, Patanjali is the first one, as far as I know.
AFAIK, Yoga can be divided into 2 broad categories:
1) Hatha Yoga
2) Raja Yoga
Hatha Yoga deals with physical postures that strengthen the body, free it from ailments and prepares the sadhak(practisioner) for the Raja Yoga.
Raja Yoga or Ashtanga Yoga has 8 parts:
1) Yama: Control(of mind).
2) Niyama: Rules
3) Asana: Posture( Control of mind through posture)
4) Pranayama: Breathing techiniques to cleanse the mind and control it.
5) Pratyahara: Control of food intake(both quality and quantity along with place and time).
6) Dharana: Memory or concentration or focus.
7) Dhyana: Meditation.
8) Samadhi: a blissful state when the practisioner experiences divinity within him or when the practisioner becomes one with god.
1) Yama: It has 5 parts:
a) Ahimsa: Non-injury. Non-injury is not non-violence of Gandhiji. Ahimsa is different in vedic/yogic tradition. Ahimsa means not injuring anyone or anything beyond the day-to-day righteous needs of a practisioner. For eg: The practisioner has to obtain vegetables by injuring the plants, but its unavoidable. Thus, it does not constitue as himsa(or torture/injury). Ahimsa is also to be practised at mental level i.e. thoughts of hurting others is also himsa.
b) Satya: Truth. Speaking facts that do not hurt virtuous is called speaking truth. When facts are spoken with an intention to hurt others, it is not considered proper. Vedas declare, " Satyam Vada(Tell Truth always). Priyam Vada(Speak pleasantly always)." Hence, the practisioner must strive to speak the truth that is pleasant. Of course, if its unavoidable, then truth must be spoken even if its unpleasant. Satya also constitues another aspect. Satyam is atma/soul and god. Denying the existance of god or soul also constitues Asatyam(false speaking).
c) Asteya: Not stealing. Stealing from others is prohibited. Further, even the thoughts of stealing from others is prohibited. Infact, coveting what does not belong to oneself is prohibited.
d) Brahmacharya: (sexual abstinence unless one is procreating with one's wife for progeny). Sexual abstinence both physical and mental unless one is in the company of one's dharmic wife and is trying for kids.
e) Aparigraha: Lack of covetousness.
2) Niyama:
a) Shaucha: cleanliness both physical and mental. Mental cleanliness constitues lack of malice, hatred and negative emotions.
b) Santosha: Happiness or contentment. Being content in all situations and not giving way to desires that lead to anger, fear, or addiction. When a desire takes root, if it is satisfied by the practisioner, then the practisioner develops affection towards it. His mind will over time again give rise to same desire and if it is satisfied, the affection strengthens. Thus, the practisioner gets addicted to it. Also, there is an associated fear that the desire may not be fullfilled in future. On the other hand, if the practisioner's desire is not fullfilled, it leads to anger and frustration. All these feelings deviate the practisioner from the goal. Hence, contentment is a prerequisite.
c) Tapas: Austerity or penance.
d) Svadhyaya: Revising the scriptures so that one does not lose the memory of dharma.
e) Ishvara Pranidhana: Self-surrender to Lord.
3) Asana: Physical postures and physical excersises.
4) Pranayama: Breathing excersises that control the mind/thoughts. The mind/thoughts that a person experiences depend on several factors both external and internal. The primary external factors are breath, food and people. The kind of food taken by a person influences a person's thoughts, similarly the kind of people that a person is accompanied by also influence his thoughts. But the most basic factor is breath. As soon as breath is held, all the chain of thoughts vanish and focus on breath alone. So, by controlling the breath one can control the thoughts or mind. Pranayama teaches one to control mind through breathing excersises.
5) Pratyahara: Just as pranayama is controlling mind with the aid of breath, pratyahara is controlling mind with the aid of food. The extreme example that food influences a person are: A person's thoughts are greatly influences when he is drunk.
Thus, the food does have the ability to influence the mind and thoughts. Foods are categorised as: Sattvic, Rajasic and Tamasic. Tamasic foods give rise to lazyness and sadistic thoughts. Tamasic foods include meat and alcohol. Thats the reason for the prohibition of meat eating for certain classes of hindus. Rajasic food gives rise to desires and aggression. Sattvic food gives rise to purer thoughts. The amount of food intake is also quite important.
6) Dharana: Concentration or focus. All the above excersises facilitate a strong concentration. As one concentrates on some subject( generally a god form or related form), one slips into Dhyana.
7) Dhyana: Meditation. This is the stage for which all the above excersises have been prescribed. A good. A good meditation is difficult to achieve without the above pre-requisites. Once, the above are met to an extent, one can meditate properly and in due course attain Samadhi.
8) Samadhi: This is the final set of stages in meditation when one starts to experience the divine within and without.
Today, people practise a very basic form of excersise which is devoid of all the spiritual, mental and physical pre-requisites that have been laid down.
Yoga, Mantra, Tantra, Yantra, Mudra...etc are inherently part of Hinduism/Santana Dharma. Yoga has been rediscovered and repackaged by some. Similarly, Mantras may be rediscovered tomorrow. But at the core these aspects are unseperable from Hinduism, further when they are alienated from the vedic message these aspects can give rise to exactly the opposite effect of what they were meant to. For example, Yoga was meant to go beyond the body, instead devoid of vedic message, Yoga has become a celebration of body!
Actually people are getting confused because of word HINDU, not knowing that what people call Hinduism is actually named 'Sanatan Dharma'. BTW it is fact that people with religious concepts with core belief in "Holier than Thou" find these types of facts very uneasy to accept.
Holier than Thou especially in regard to what they consider as pagan...