Yoga Nidra means Yogic Sleep. It is a state of conscious Deep Sleep. In Meditation, you remain in the Waking state of consciousness, and gently focus the mind, while allowing thought patterns, emotions, sensations, and images to arise and go on. However, in Yoga Nidra, you leave the Waking state, go past the Dreaming state, and go to Deep Sleep, yet remain awake.
Yoga Nidra brings an incredible calmness, quietness and clarity. It is one of the deepest of all meditations, leading awareness through many levels of mental process to a state of supreme stillness and insight.
In a Yoga Nidra class, you are lying down, sometimes supported by props (bolsters, blankets, etc...) and are guided through specific relaxation techniques and into a Yoga Sleep state. It is important to feel safe and secure while in this state and sometimes is most effective with a friend or beloved experiencing it with you. While Yoga Nidra is a state that is very relaxing, it is also used by Yogis to purify the Samskaras, the deep impressions that are the driving force behind Karma.
As a state of conscious Deep Sleep, Yoga Nidra is a universal principle, and is not the exclusive domain of any more recent teachers or traditions.