The Yoga Sutra, widely regarded as the authoritative text on yoga, is a collection of aphorisms, outlining the eight limbs of yoga. These "threads" (as sutra translates from Sanskrit) of wisdom offer guidelines for living a meaningful and purposeful lIfe. Patanjali is the sage who wrote the sutras.
The truth is that nobody really knows much about Patanjali. We don't even know exactly when the sage lived. Some practitioners believe he lived around the second century BCE and also wrote significant works on Ayurveda (the ancient Indian system of medicine) and Sanskrit grammar, making him something of a Renaissance man.