Patanjali's eight-limbed path, also known as ashtanga, forms the basis of classical yoga. This system is explained in the Yoga Sutras, and though neither the first nor the only ancient writing on yoga, the sutras are so important because they provide a natural progression for spiritual evolution.
For most of us earthly beings, yoga's ultimate culmination of self-realization cannot be attained on demand. Patanjali understood the obstacles of the human psyche, and gifted us with a system to naturally and gradually prepare our morality, body, mind, and senses for yoga's highest goals. Each limb of his path builds upon those before it, and to skip any of the steps would be a major obstacle—if not the demise—to further progress.
The eight limbs of yoga start with the yamas and niyamas, laying the groundwork for the moral conduct needed to guide us on our yogic journeys. The limbs end with samadhi: the ultimate goal of yoga.
The yamas are five ethics that primarily deal with our relationships with others. They set us up for a harmonious existence in society. They include:
The niyamas are five principles that are oriented toward individual conduct and discipline. They encourage us to adopt a pure lifestyle and mindset to promote spiritual growth. They include:
Modern yoga gives this limb the most emphasis, but it's merely one of eight other steps! Asana is the physical practice of yoga, which is meant to purify the physical body and promote its long-lasting health. Through asana, we learn to find effortlessness in effort. In a literal sense, we learn to find a steady and comfortable posture that can be sustained for a long period of time in meditation.
Pranayama, or breath regulation, forms a bridge between the three externally-centered limbs above and the four internally-centered limbs that follow. After preparing the body through asana, we control and expand the breath through pranayama to increase our life force. Pranayama clears the mind of distraction and ignorance, preparing it for the next stages of yoga.
Pratyahara is the practice of sensory withdrawal. After the body and its energy are mastered through the previous four limbs, the senses need to be mastered. When the mind is no longer a slave to the senses, it can progress on the spiritual path.
Dharana means concentration, and this step is imperative for the next limb: meditation. Without concentration, meditation is impossible. And to be able to properly concentrate, we must first gain control over the body, prana, senses, and mind—the goal of the five preceding steps. We can work on dharana by focusing the mind on one object, image, or idea.
Dhyana is the practice of meditation. Meditation spontaneously arises through the uninterrupted flow of concentration. Hence, dharana must first be mastered before this limb can be approached.
Samadhi comes through sustained meditation and is the highest goal of yoga. In this stage, the sense of 'I' or 'me' is surpassed. The divine nature is seen in everything. There are various levels of samadhi, but it's the higher yogic samadhis that are needed to attain self-realization.
The limbs do build on one another, but their progression isn't meant to be rigid. For example, you might begin pranayama before you have mastered asana, but you should follow the general progression to avoid obstacles on your path.