Yin
Yin yoga is a meditative practice that allows your body to become comfortable in a pose without doing any work (strength). It is also called Taoist yoga, and focuses on lengthening the connective tissues within the body. It is meant to compliment Yang yoga, or muscle-forming yoga practices. The poses are held with the muscles fully relaxed, allowing time and gravity to deepen the stretch and target the fascia. The time spent holding these asanas is similar to meditation.
If Yang is active, then Yin is passive, meaning the muscles are allowed to relax with gravity and rest. This is usually performed with the assistance of props, and there is little or no music in class.