Astanga Yoga falls under the category of Sankhya Yoga (Way of realizing God by Meditation). It consists of eight stages. The following are these eight stages.
1. Yama (Social Discipline)
Yama means restraint or abstension. It refers to five moral practices. They are :Ahinsa (nonviolence), Satya (truthfulness), Asteya (nonstealing), Brahamcharya (moderation in sex) and Aparigraha (nonacquisitiveness).
Ahinsa (nonviolence) means not to hurt any creature mentally or physically through mind, speech or action.
Satya (truthfulness) is the presentation of the matter as perceived with the help of sense organs.
Asteya (nonstealing) means not to covet and acquire physically, mentally or by speech other's possessions.
Brahamcharya (moderation in sex) does not mean lifelong celibacy, but moderation in sex between married couples.
Aparigraha (nonacquisitiveness) means abandoning wealth and means of sensual pleasurs.
2. Niyama (Individual Discipline)
It means rules of conduct towards oneself. It consists of certain disciplines which are both physical and mental.
These are five in number. Shaucha (cleanliness), Santosha (contentment), Tapas (austerity), Svadhyaya (self-study) and Ishvara Pranidhana (surrender to Braham).
Shaucha (cleanliness) means internal and external purification of the body and the mind.
Santosha (contentment) is a state of mind by which one lives happily and satisfied in congenial or uncongenial atmosphere.
Tapas (austerity or penance) is the conquest of all desire or sensual pleasures by practising purity in thought, speech and action.
Svadhyaya (self-study/discussion of Braham related knowledge) means exchange of thoughts in order to secure purity in thought and accomplish knowledge.
Ishvara Pranidhana (surrender to Braham) consists of pure devotion to GOD and surrender of all actions to Him.
3. Aasana (postures)
It means holding the body in a particular posture to bring stability to the body and poise to the mind. The practice of Aasana brings purity in the tubular channels, firmness to the body and vitality to the body and the mind.
4. Pranayama (Breath Control)
The literal meaning of the Pranayama is Breath Control. The aim of practising Pranayama is to stimulate, regulate and harmonize vital energy of the body. Just as bath is required for purifying the body, so also Pranayama is required for purifying the mind.
5. Pratyahara (Discipline of senses)
The extroversion of the sense organs due to their hankering after worldly objects has to be restrained and directed inwards towards the source of all existence (Braham). This process of drawing the senses inwards is Pratyahara6. Dharna (Concentration)
It means focussing the pure mind on Braham. The practice of Dharana helps the mind to concentrate on a particular object.
7. Dhyana (Meditation)
When one sustains and maintains the focus of attention through Dharana unbounded by space and time, it becomes Dhyana. Deep concentration destroys the Rajas Gunas and Tamas Gunas of the mind and develops Satvika Gunas. Please read Bhagvadgita to understand the Gunas (charasterstics) of the mind and body.
7. Samadhi (Self-realisation)
The eighh and final stage of Yoga is Samadhi. At this stage one's identity becomes both externally and internally immersed in Braham. This is like salt mixing in sea. The meditator, the act of meditation, and the object meditated upon all three shed their individual characterstics and merge with one single vision of the entire cosmos and become Braham only, which is the true nature of every object. Supreme happiness free from pleasure, pain or misery, is expeerienced. Samadhi is the climax of Dhyana.
The group of last three stages, namely, Dharana, Dhyana and Samadhi is called Samyam and is referred to as Internal Yoga in the Science of Yoga. These come by practice. And one who is able to realise oneself is eternal even in this body, one gets Mokcha. Please read Bhagvadgita to understand it more.
The first five stages- Yama, Niyama, Asana, Pranayama and Pratyahara constitute the External Yoga. If all these five stages are practised and followed in life, virtues like morality develop in human. Besides there is allround progress in human life, physically, intellectually and spiritually and human attains physical fitness and mental equanimity.