Which is the key belief of buddhism?

Buddhism, like most of the great religions of the world, is divided into many different traditions. However, most traditions share a common set of fundamental beliefs.
One fundamental belief of Buddhism is often referred to as reincarnation -- the concept that people are reborn after dying. In fact, most individuals go through many cycles of birth, living, death and rebirth. A practicing Buddhist differentiates between the concepts of rebirth and reincarnation. In reincarnation, the individual may recur repeatedly. In rebirth, a person does not necessarily return to Earth as the same entity ever again. He compares it to a leaf growing on a tree. When the withering leaf falls off, a new leaf will eventually replace it. It is similar to the old leaf, but it is not identical to the original leaf.
After many such cycles, if a person releases their attachment to desire and the self, they can attain Nirvana. This is a state of liberation and freedom from suffering.