The Yoga Sutras define yoga, diagnose what blocks it, and prescribe a path. The prescription — ashtanga, eight limbs — runs from ethical foundation (yama, niyama) through posture (Asana) and breath (Pranayama) to concentration, meditation, and Samadhi.
The second verse is often quoted as the whole work in miniature: yogash chitta vritti nirodhah — yoga is the cessation of the modifications of the mind.