Yoga is classified into 8 steps:
When body and mind are cultivated properly, the seeker becomes fit to enter into transcendental meditation. Nerve currents begin to pass throughout the body from the central nervous system (shushumna), and from the entire body to the central nervous system. In the state of meditation, the chest, neck and head must remain in one straight line. Let the whole weight of the body be supported by the ribs and by the spine. One cannot tend to noble thoughts if one's body is bent in any direction.
That part of Yoga which deals with various physical and mental exercises, breathing exercises and breath control is called Hatha Yoga. Its aim is to eliminate elements of sickness and to make the body strong and fit for meditation. That part of Yoga which deals with various chants and the repetition of mantras is called Mantra Yoga. That part which deals with the transformation of the energy of the individual mind into cosmic energy, and the consequent absorption of the individual soul into the cosmic Soul is called Laya Yoga. That part of Yoga which, by scientific methods, integrates every manifestation of the universe into the Self and reveals the presence of the Self in every object and being of the universe is called Raja Yoga.
Pranayama is not, as many think, only breathing exercises. No doubt breathing exercises are the stepping-stone to Pranayama, but it is not the stopping-stone. "Prana'' means energy and life; "ayama'' means expansion. Thus expansion of the individual energy, individual life and individual self into the cosmic life, cosmic energy and cosmic Self is Pranayama.
The whole universe is composed of two materials: Akasha (ether) and Prana (energy). Akasha is the omnipresent all-penetrating existence. Everything that has form, everything that is the result of combination is manifested out of Akasha. In short, it is the entire sense-perceived universe. It is itself so subtle that it cannot be perceived by the senses; it can only be perceived when it becomes gross. At the beginning of creation, there is only this Akasha. At the end of the cycle, the solids, liquids and gasses all melt into the Akasha again, and the next cycle of creation again proceeds out of Akasha.
By the power of Prana, Akasha manufactures itself into this sense-perceived universe. Just as Akasha is the infinite, omnipresent material of this universe, so is Prana the infinite omnipresent manifesting power of this universe.
At the beginning and end of the cycle of creation all the forces which are operation in this universe resolve back into the Prana. Akasha is static energy, whereas Prana is dynamic energy, and the Self is that in which both forms of energy are resolved. The result of Pranayama is to resolve static and dynamic energy into the Self.
The word Samadhi is used in two senses: general and specific. In the general sense, it is the transformation of the individual self into the universal self; the transformation of the mind and matter into energy and then into the Self. In the special sense, there are two classes of Samadhi in which all division and subdivisions of Samadhi are included.
First is "Samadhi with effort'', which means continuous practice day and night. If practice is stopped, then karmas and old habits will attack the practitioner again. This Samadhi is like the winter in which, due to the cold, every vegetable and tree seems to be dead; but in the spring, everything returns to life. So, with this Samadhi, if one ceases to practise, every habit and every karma springs up once again.
Second, there is "effortless Samadhi'' which is the result of the above mentioned Samadhi. The individual self is submerged into the supreme Self and all karmas are totally destroyed. It can be likened to fried seeds which can never sprout again. A person who has achieved this state is called an enlightened one, and is the moving image of God.
These 8 steps of Yoga are sub-classified into 2 main groups. The first group consists of the first five steps, and the second group consists of the last three. The first group deals with matter and mind, hence it is called external. The second group deals with the mind and soul, and hence it is called internal.
The practice of all 8 steps is absolutely necessary. One may read and listen to lectures on Yoga, but one will not be successful unless one practises. By attending discourses and reading many books on Yoga, one can become a giant intellectually, but one cannot become enlightened unless these teachings are transformed into one's very blood.