Yin Yoga postures are passive postures, mainly on the floor and the majority of postures equal only about three dozen or so, much less than the more popular yang like practices. Yin Yoga is unique in that you are asked to relax in the posture, soften the muscle and move closer to the bone.
While yang-like yoga practices are more superficial, Yin offers a much deeper access to the body. It is not uncommon to see postures held for three to five minutes, even 20 minutes at a time. The time spent in these postures is much like time spent in meditation, students are often talked through the postures as if they were trying to meditate. While in a Yin class you might notice similar postures to a yang class except they are called something else, on a basic level this is to help the students mind shift form yang to yin, active to passive.