Heaven and hell
Nirvana and samsara are one and the same, though seen from different viewpoints. The wise see nirvana, and rest in it. The ignorant see and struggle with samsara. Samsara is the world of happiness and suffering; it is the world of change. Nirvana is beyond both happiness and suffering; it is constant.
Samsara
Happiness is the transition from one form of suffering to another. Suffering is the transition from one form of happiness to another. Samsara is made of these transitions.