What Is Enlightenment?
The only true answer to the question "What is enlightenment?" is to realize enlightenment. Short of that, we must come up with provisional answers.
The English word enlightenment sometimes refers to heightened intellect and reason. This kind of enlightenment is a quality that can be cultivated or possessed. But enlightenment in the Buddhist sense is not a quality, and no one can possess it.
For this reason, many Buddhists are cautious about using the word enlightenment. The original Buddhists used the word bodhi, which means "awakened." The word Buddha is derived from bodhi and means "the awakened one." To be enlightened is to be awake to a Great Reality that most of us do not perceive. Some Zen teachers use the word realized in place of enlightened.
What Is Nirvana?
The Buddha told his monks that Nirvana cannot be imagined, and so there is no point speculating what it is like. Even so, it is a word that Buddhists use, so it needs some kind of definition.
The word Nirvana means "to extinguish," such as extinguishing the flame of a candle. Some schools explain Nirvana as a state of bliss or peace, and this state may be experienced in life, or it may be entered into at death. Other schools define it as oneness with the Absolute.