How is Nibbana different from the everlasting heaven, as taught in some religions?
Nibbana (Nirvana in Sanskrit) is not a realm of existence or heaven. It is a state attained by the complete liberation from dukka, by the complete elimination of the root cause, which is craving (tanha). A person who has attained the state of Nibbana will no longer acquire kamma that keeps the cycle of life going, hence there will no longer be rebirth.
Nibbana cannot be described in terms of our normal experiences, which are so limited. Therefore Nibbana is generally described in negative terms such as 'extinction of desire', 'extinction of hatred', 'Unconditioned' (conditions that produce cause and effect), 'Cessation' (of continuity) etc.
A person who has attained Nibbana experiences a state of supreme joy that comes from being completely free from attachment, free from hatred, free from delusions.
Nibbana can be attained in this life, not after death. As this is a long and difficult process, the success of attainment in this life very much depends on whether the person had practised the path in his/her previous lifetimes. The Buddha and many of his followers attained this state during their own lifetimes.