Nirvikalpa Samadhi, and Pancha Koshas
Vikalpam means difference, divide etc. 'Nir' means without. Nirvikalpa hence means without any difference.
In savikalpa samdhi, though the soul unites with the God, it still feels its identity. In nirvikalpa, it senses nonduality. Hence this is supposed to be highest stage in Ashtanga Yoga (as propounded by Sri Patanjali).
This state is indeed extremely challenging to obtain.
We are built of five 'koshas' or sheaths (or layers). (maya is not maaya / delusion.)
1. 'Anna'maya kosha
2. 'Praana'maya kosha
3. 'Manon'maya kosha
4. 'Gnana'maya kosha
5. 'Ananda'maya kosha
'Annam' means rice / food. So, our physical body is referred to as 'anna'maya kosham. This sheath can be contained by Yogic postures / asanas / Hatha yoga kriyas.
'Praana' implies the 'praana'shakthi. Praana is usually mistaken for air. Air can be compared to a horse and 'praana' to be the rider. In India, when people die, it is normal to hear the quote 'praana' has left. It is 'praana' that sustains life. This sheath is obiously subtler than 'anna'maya kosha. This can be conditioned by pranayama (Kriya Kundalini Pranayama).
Manas implies mind. So, the sheath subtler than praana is the mind. So, 'manon'maya kosha imlies our mind. You would have noticed that when we are tense we breath fast and when we are relaxed, we breath deep and slow. Thus, the mind has effect on Praana. So, the reverse is also true. When we breath slow and easy, the mind gets controlled. This is the principle of pranayama. This sheath can be conditioned by Dhyana (meditation) Kriyas.
Gnaana implies intellect. The sheath subtler than mind is intellect. Intelligence usually should control the mind. But, mind is usually not controlled and is driven by the senses. When the intellect governs the mind, only good things happen. This sheath can be conditioned by Kriya Mantra Yoga by repeating mantras and by
contemplating on vedic truths.
The subtlest of all is the 'ananda'maya kosha or the bliss sheath. This is indeed the soul. This can be conditioned / realised by Kriya Bhati yoga.
Only when all the five koshas are transcended, AND, the ego is completely eliminated, can one attain Nirvikalpa Samadhi
 This is the edited form of a post in my Yahoo Group - Murugavataar Babaji
 Terminologies used are based on Kriya Yoga