Yang Yoga targets the muscles with aggressive stretching postures and breathing exercises. Yin Yoga is the counterbalance to the practice of Yang Yoga, and is sometimes referred to as yoga for the joints.
This yoga form begins with relaxed muscles and targets the ligaments and joints that asana styles don't normally exercise. It involves postures that are held for longer periods of time. This stretches and strengthens connective tissues that are much deeper than superficial or muscular tissues.
Yin and Yang poses balance each other out and give the body the full exercise it truly needs.