Yoga is the state where in there is no sankalpa-vikalpa (thought or doubt). Yoga is the control of mind and its modifications. Yoga is the equal state between jivatma (soul) and paramatma (God). The word ‘Yogi’ signifies an aspirant or a student in the path of yoga, as well as a full-blown developed adept in yoga. He who is fully established in the highest nirvikalpa samadhi (super-consciousness) is a yogi. He who practicses yoga is also a yogi. A ‘raja yogi’ is otherwise known as ‘dhyana yogi. Dhyana means meditation. It is the continuous flow of one idea of God. Raja yoga aims at controlling all thought waves or mental modifications. It concerns with the mind, its purification and control. Hence it is called ‘raja yoga, king of yogas.’ It is otherwise known as ashtanga yoga (yoga with eight limbs).
A sutra means a thread or aphorism or terse saying. Each sutra is pregnant with high, sublime ideas. Without the help of commentary, It is difficult to understand the meanings of the aphorisms. Just as flowers or pearls are studded in a string or a thread, even so, philosophical or spiritual ideas are studded in a sutra or thread.
Food (ahara), sleep (nidra), fear (bhaya), and coition (maithuna) are common to both animals and human beings. But man has got intelligence, power of discrimination (viveka) and power of enquiry (vicara sakti); with the help of these special powers, he can realise his self and can know his true nature. Even devas (gods) are envious of human beings, because deva-yoni (divine species) is mere bhoga-yoni (enjoyment). They can enjoy only with a daivic (divine) body. Man has got both karma-yoni (activity) and bhoga-yoni. He can do nishkamya karma yoga and attain jnana through citta-suddhi (purity of mind). He can take recourse to various yogic practices for spiritual unfoldment and can become a full-blown yogi in this birth. Horses and dogs possess minds. But they have neither discrimination nor intelligence nor vicara sakti. Hence it is not possible for them to attain freedom.
You will find in Yajnavalkya Smrti that Hiranyagarbha was the original teacher of yoga. Patanjali Maharishi is only a compiler, or explainer, of the yogic principles and tenets taught and practiced by Hiranyagarbha and others.