Yin and Yang
Yin Yoga is based on the Taoist concept of yin and yang, opposite and complementary principles in nature. Yin is the stable, unmoving, hidden aspect of an object, while Yang is its changing, moving, revealing aspect. These two objects always coexist, and everything in nature can be described in terms of its yin and yang.
All forms of yoga can be described as Yin or Yang based on which tissues of the body are being targeted. A practise that focuses on gentle traction and the stretching of ligaments, tendons and fascia (connective tissue) is Yin yoga and a practise that focuses on exercising and stressing the muscles is Yang yoga.