Parinirvana is final nirvana. It is understood within the story at the end of the life of the Buddha.
In this story as the Buddha is approaching the end of his biological life after many years of spreading the Eightfold Path. The Buddha explains to his most beloved disciple that if a monk were to ask a Buddha to extend his life forever, the Buddha has it within his power to do so. It seems however, that the disciple fails to appreciate the Buddha's words, and with this he dies with some words of parting; words which interestingly are melancholy to say the least.
So what then is parinirvana? Well since nirvana is the extinguishing of the self, how can there be anything left to cease? It is the fact that even though a being can achieve its annihilation, it has a choice to cease to be forever, or return in the role of a Bodhisattva for a time. At the end of every Bodhisattva's life he has a choice as to reenter samsara or walk onward into the darkness and light.
Parinirvana then is simply the cessation of consciousness and biological life coupled with the decision to pass out of existence completely, as opposed to returning as a Bodhisattva after one has become enlightened.