The term "ashtanga" comes from the Yoga Sutras of Patanjali, the compendium referring to the classical practice of the 8 (ashta) limbs (anga) of yoga. It consists, then, of the eight progressive stages on the path to self-realization. A path of discovery built perfectly as it offers all the necessary tools, it is a kind of compendium of inner discovery encoded for all. Regardless of age, circumstance, disease state, sex, gender, options, ideals. To achieve where, after all, we all desire. Happiness, conscience, equanimity.
We can divide the 8 limbs into two parts.
The first four are the external pillars that include both inner (yamas) and social (nyamas) behaviors; Two technical chapters, the asanas or postures (the third in the list) and the pranayama, the breathing techniques. Without observance of these steps there is no yoga. There may be a more beautiful and flexible body but there is no understanding of the transformation.
The final 4 come naturally after the first ones are established. They are internal practices: pratyahara (senses withdrawal), dharana (concentration), dhyana (meditation) and Samadhi (contemplation).