In Taoist philosophy, the qualities of Yin are dark, calm, passive and deep. Based on these principles, Yin Yoga uses passive postures held for longer periods of time (1-5 minutes or more) to allow for deep opening in the connective tissue of the hips, pelvis and spine. Holding Yin poses for longer periods of time really allows you to sink into the pose, to get to know where you may be feeling resistance, and to listen more closely to your body and mind. Focusing on stretching the connective tissue of the joints rather than the muscle makes it easier to sit in meditation.
Yin Yoga is often confused with restorative yoga because the poses are held for long periods of time. They are different, however, in that the main goal of restorative yoga is relaxation through the support of props, whereas the main goal of yin yoga is the opening of the connective tissues surrounding the joints, with less focus on relaxation.
Yin Yoga can be practiced by any level of student and can be very beneficial for increasing flexility in muscles surrounding the joints (such as the hips) that tend to lose mobility with age. However, Yin yoga is not to be confused with Yin-Yang Yoga, which will alternate quiet restorative poses with intense flow sequences and require a higher level of fitness.