Patanjali's Yoga Sutra opens with a definition rather than a prayer: yogash chitta vritti nirodhah, yoga is the stilling of the fluctuations of the mind. Everything the text describes is a method for reaching that state, not a theology and not, in its classical form, a set of postures for their own sake. The system takes its metaphysics from Samkhya, two irreducible principles, purusha, pure consciousness, and prakriti, nature in all its forms including thought itself, and adds a discipline for getting consciousness to stop mistaking itself for the activity it merely observes.
That discipline is the eight limbs, ashtanga. Two are ethical, restraint from harm and dishonesty, and observance of purity, contentment and study. Three are bodily and attentional, posture, breath control and withdrawal of the senses. Three are progressively deeper states of focus, concentration on a single object, sustained meditation, and finally absorption, samadhi, in which the distinction between observer, observing and observed collapses. The sequence is cumulative, each limb a precondition the next depends on.
The one place classical Yoga visibly departs from Samkhya is Ishvara. Samkhya's universe generally runs without a creator, explained by purusha and prakriti alone. Patanjali instead names Ishvara, a special purusha never touched by affliction, action or its results, and offers devotion to it, Ishvara-pranidhana, as one route toward the goal the other seven limbs pursue by different means. It is not a creator god, but its presence marks classical Yoga as a distinct position rather than Samkhya with exercises attached, and it is this system, argued over by commentators from Vyasa onward, that the word darshana names here.