According to Patanjali's Classical Yoga, Yoga has what are called eight limbs, eight parts -- in Sanskrit, "Ashtanga," (ash (eight) tanga (limbs)).
And here's where it may start to get a little confusing for some people, because there is also a popular system of physical culture yoga called "Ashtanga Yoga." But Pattabhi Jois' Ashtanga Yoga, part of what's called the Mysore School, is not the same as the Ashtanga Yoga of Maharishi Patanjali, as described in the Yoga Sutras.
Here we're talking about the Ashtanga Yoga of Maharishi Patanjali, described in his Yoga Sutras, composed almost two thousand years ago, also called Classical Yoga.
For Patajali, the Eight limbs are not eight separate practices. They are aspects, dimensions, attributes of all Yoga practice, or of any particular Yoga practice. Patanjali seems mainly focused on what we call inner practice, meditation practice: the mental and emotional cultivation of total unresisted flow to foster a state of total freedom he calls "kaivalya."
It is worth noting that there is virtually no mention of any physical practice of Yoga anywhere in the Yoga Sutras. What we know as Asana or Vinyasa practice is a relatively modern invention. It seems that originally, Yoga practice focused mainly on training the mind and heart to achieve this state of Yoga, total harmony with Nature. Yoga-on-the-mat came later.
For Patanjali, Yoga practice entails the concurrent development of all of the eight limbs together. The Eight limbs describe the inner dimensions of the practice, as opposed to the external aspects of the practice. Properly practiced and understood, they align mind, heart and spirit to support, energize, complete and fulfill the external practices of Yoga (physical and behavioral practices).
It's worth noting that while Patanjali focuses mainly on meditation practices, the eight limbs he describes equally apply to any of the other formal Yoga practices that came later on -- to Vinyasa, or Bhakti, or Jnana or Karma or Raja Yoga practice. All Yoga practice benefits when it conforms to the eight limbed model.
The Eight Limbs of Yoga are as follows:
1. Yama (principles or precepts) -- a list of five concepts that comprise the main principles (guidelines) of Yoga Practice. All practice should conform to these five principles. They are non-aggression, acceptance, non-comparison, self-sufficiency and non-clinging. These are to be learned, understood and implemented.
2. Niyama (observances or attributes) -- a list of five concepts that describe the main manifestations of successful Yoga practice, things we observe in ourselves and others as a result of practice, things that show us we're on the right track. They are purity, contentment, radiance, self-understanding and surrender to God or Nature. Again, we learn, observe and understand these principles.
3. Asana (foundation) refers to one's attitude or disposition on every level: physical, mental, emotional and spiritual. It is one's basic disposition, foundation, or framework of being, and it should be steady and feel comfortable, whether we're on our mat, in our meditation, in the supermarket checkout line, or anywhere else in our everyday life.
Patanjali's use of the word Asana is not to be confused with the physical postures of Hatha Yoga, also called Asanas. His use of the word Asana is much bigger, much more fundamental and all-encompassing.
Yama, Niyama and Asana form the basis for the next two limbs. They are the platform on which the next two limbs rest.
4. Pranayama (energy control) Prana is life-energy; it is also the breath. (Much like the the two uses of inspiration in English: referring to both the breath and the spirit.) Achieving balance in the flow of energies (experienced mostly as attitude, mood, motivation and vitality) throughout one's system is essential to Yoga. Working with the breath is only the beginning. Next we learn to work with emotion and then intention to produce a steady state of even-temperedness and equanimity relative to loss and gain (samapatti). The three stages of pranayama are mood balance, energy balance and intentional energy attunement (shaktipata).
5. Pratyahara (de-identification) Refers to the ability to withdraw from mental/emotional identification with the objects of perception.
We could also understand it as disengagement from the kleshas, the barriers to Yoga: identification with the finite, the temporal (avidya); identification with the individual ego (asmita); attachment and aversion, (raga and dvesha); and clinging to life (abinivesha). Patanjali lists these five "afflictions" as the chief barriers to Yoga.
But on a simpler level, Pratyahara means being able to shift mental frames of reference to allow for free movement of attention, allowing for freedom regarding mental/emotional attitudes and choices. It means a mind with no fixed attitudes or ideas, free movement of attention.
Pranayama and Pratyahara are necessary to stabilize and free up the mind and attention in support of the next three limbs. They make the last three limbs possible.
6. Dharana (focus) the ability to naturally and easily hold one's attention on a specific point; to hold a specific attunement of mind and heart energy. We use a mantra or mental theme to develop and strengthen Dharana.
7. Dhyana (attunement) The ability to maintain that focus so that rapport and polarity develops between the individual and the universal, allowing for a deeper, more profound experience of attunement and energy exchange between the two.
Dhyana also refers to the systematic development of alignment between mind, heart and spirit. This experience must be a part of any and every practice of Yoga -- vinyasa, meditation, service, study or devotion. The proper use of mantra is an easy way of learning to achieve and maintain a state of dhyana.
8. Samadhi (absorption) The collapse of subject/object separation and the experience of unity or oneness between observer and observed. Profound intimacy and inter-subjectivity. Total flow. There are many different levels to samadhi, but the two main distinctions are samadhi with thought (savikalpa) and samadhi without thought (nirvikalpa). Both are the product of full engagement of all eight limbs in practice.
According to Patanjali, the last three limbs are three parts of one process he calls Samyama. Becoming able to enter into Samyama (total unrestricted flow) is the fundamental objective, the main mechanism of all formal practice of Yoga. It's where the real benefits start to happen. All else is preliminary to that.
Each of the eight limbs (eight aspects, attributes or dimensions) of practice must be present to some extent in order to fulfill the practice and the promise of Yoga -- physical and material well-being; enjoyment and happiness; life-supporting and life-supported behavior; and liberation. They must all be brought forth together for Yoga to take place.
They are not to be understood as separate practices, steps, or rungs on a ladder, but rather as eight aspects of one complete practice -- eight limbs of one tree. All eight limbs must be active in practice if the promise of Yoga is to be fulfilled.
Any and all formal practice, vinyasa or meditation, for example, must rest on, include and conform to the eight limbs in order to be complete and effective.
We learn and understand them one at a time; we practice them all together. This is Ashtanga Yoga. This is Patanjali's eight limbed practice.