"But what is Nirvana? The only reasonable reply to give to the question is that it can never be answered completely and satisfactorily in words, because human language is too poor to express  the real nature of the absolute truth or ultimate reality which is Nirvana. Language is created  and used by masses of human beings to express things and ideas experienced  by their sense organs and their mind. A supramundane  experience like that of the absolute truth is not such a category. Therefore there cannot be words to express that experience.

Nirvana is generally expressed in negative terms - a less dangerous mode perhaps. So it is often referred to by such negative terms as extinction of thirst, uncompund, unconditioned, absence of desire, cessation, blowing out or extinction.

Referring to Nirvana the Buddha says: O bhikkus, there is the unborn, ungrown and unconditioned. Were there not the unborn, ungrown and unconditioned, there would be no escape for the born, grown and conditioned.

Nirvana is definitely no annihilation of self, because there is no self to annihilate. If at all, it is the annihilation of the illusion, of the false idea of self.

Nirvana is neither cause nor effect. It is beyond cause and effect. Truth is not a result nor an effect. It is not produced like a mystic, spiritual, mental state. Truth is. Nirvana is.

What is there after Nirvana? This question cannot arise, because Nirvana is the ultimate truth. If it is ultimate, there can be nothing after it. If there is anything after Nirvana, then that will be the ultimate truth and not Nirvana.

If there is no self, who realizes Nirvana? It is the thought that thinks, there is no thinker behind the thought. In the same way, it is wisdom, realization, that realizes. There is no other self behind the realization.

Nirvana is beyond logic and reasoning."

What the Buddha taught - p.35-43