Yoga Nidra is a form of guided relaxation that goes very deep. It is practiced laying down on your back while the facilitator guides you for relaxing each point of the body, and then through a mental awareness exercise that ultimately balances senses and mind to realize a deep state of inner awareness. This deepest form of relaxation can only occur when we withdraw our senses from the outside world and occurs more naturally if we are sweetly guided through each "step" of moving inwards.
You can practice Yoga Nidra using this recording from George Anthony. Make sure you have a comfortable and quiet place to lay down and sit to listen to the instructions.