What is taught around the world as yoga is one limb of an eight-limbed system, and not the limb the classical texts spend their time on.
Patanjali's Yoga Sutras, compiled somewhere in the early centuries of the common era, set out the sequence. Yama, restraints: non-harming, truthfulness, not stealing, continence, non-possessiveness. Niyama, observances: purity, contentment, austerity, study, surrender to God. Asana, posture. Pranayama, regulation of breath. Pratyahara, withdrawal of the senses. Dharana, concentration. Dhyana, meditation. Samadhi, absorption. The first two are ethical prerequisites rather than optional additions, and the text is explicit that the later limbs do not work without them.
Asana in the Sutras means a seat: a posture that is steady and comfortable and can be held a long time without distraction. It does not describe a sequence of shapes, and the text says almost nothing further about it. The elaborate postural repertoire belongs to the much later hatha yoga literature, from roughly the eleventh century onward, which developed a physical technology of postures, locks, seals and breath retention aimed at manipulating an internal anatomy of channels and centres.
The goal is also not what most modern practice implies. Classical yoga aims at kaivalya, isolation: the seer standing entirely apart from the movements of mind. That is a dualist framework, close to Samkhya philosophy, in which spirit and matter are permanently distinct. It is not the non-dual Vedanta commonly used to explain yoga today, where the self and the absolute turn out to be one. Substituting the second for the first is a genuine distortion rather than a simplification, and it is made constantly.