The eight limbs of Yoga

Ashtanga yoga consists of the following limbs: The first five are called external aids to Yoga (bahiranga sadhana)

Yama refers to the five abstentions: how we relate to the external world. (These are the same as the five vows of Jainism).

Ahimsa: non-violence, inflicting no injury or harm to others or even to one's own self, it goes as far as nonviolence in thought, word and deed.

Satya: non-illusion; truth in word and thought.

Asteya: non-covetousness, to the extent that one should not even desire something that is his own; non-stealing.

Brahmacharya: abstinence, particularly in the case of sexual activity. Also, responsible behavior with respect to our goal of moving toward the truth. It suggests that we should form relationships that foster our understanding of the highest truths. "Practicing brahmacharya means that we use our sexual energy to regenerate our connection to our spiritual self. It also means that we don’t use this energy in any way that might harm others."

Aparigraha: non-possessiveness; non-hoarding

Niyama refers to the five observances: how we related to ourselves, the inner world.

Shaucha: cleanliness of body and mind.

Santosha: satisfaction; satisfied with what one has.

Tapas: austerity and associated observances for body discipline and thereby mental control.

Svadhyaya: study of the Vedic scriptures to know about God and the soul, which leads to introspection on a greater awakening to the soul and God within,

Ishvarapranidhana: surrender to (or worship of) God.

Asana: Discipline of the body: rules and postures to keep it disease-free and for preserving vital energy. Correct postures are a physical aid to meditation, for they control the limbs and nervous system and prevent them from producing disturbances.

Pranayama: control of breath. Beneficial to health, steadies the body and is highly conducive to the concentration of the mind.

Pratyahara: withdrawal of senses from their external objects.

The last three levels are called internal aids to Yoga (antaranga sadhana)

Dharana: concentration of the Chitta upon a physical object, such as a flame of a lamp, the midpoint of the eyebrows, or the image of a deity.

Dhyana: steadfast meditation. Undisturbed flow of thought around the object of meditation (pratyayaikatanata). The act of meditation and the object of meditation remain distinct and separate.

Samadhi: oneness with the object of meditation. There is no distinction between act of meditation and the object of meditation. Samadhi is of two kinds:

Samprajnata Samadhi conscious samadhi. The mind remains concentrated (ekagra) on the object of meditation, therefore the consciousness of the object of meditation persists. Mental modifications arise only in respect of this object of meditation.

This state is of four kinds:

Savitarka: the Chitta is concentrated upon a gross object of meditation such as a flame of a lamp, the tip of the nose, or the image of a deity.

Savichara: the Chitta is concentrated upon a subtle object of meditation, such as the tanmatras

Sananda: the Chitta is concentrated upon a still subtler object of meditation, like the senses.

Sasmita: the Chitta is concentrated upon the ego-substance with which the self is generally identified.

Asamprajnata Samadhi superconscious. The Chitta and the object of meditation are fused together. The consciousness of the object of meditation is transcended. All mental modifications are checked (niruddha), although latent impressions may continue.

Combined simultaneous practice of Dhāraṇā, Dhyāna and Samādhi is referred to as Samyama and is considered a tool of achieving various perfections, or Siddhis. But as stated above, siddhis are but distractions from Kaivalaya and are to be discouraged. Siddhis are but Maya. The purpose of using samadhi is not to gain siddhis but to achieve Kaivalya.

http://en.wikipedia.org/wiki/Yoga_Sutras_of_Patanjali