The Buddha's key teachings are the Four Noble Truths and the Eightfold Path. Buddhists accept and follow these teachings in order to achieve enlightenment. There are differences within Buddhist traditions concerning how they understand ideas about life after death.
Theravada Buddhists strive to become arhats – enlightened people who have achieved Nirvana, who will not be born again. They teach that it is very likely that only monks and nuns can ever achieve this because:
Mahayana Buddhists strive to become a bodhisatta - a person who wants to become enlightened but who decides to be reborn and help others attain enlightenment.
Some Mahayana Buddhists believe that enlightenment can be achieved in one lifetime, and that this is possible by a layperson (someone who is not a monk or a nun). They make a vow stating that they will try to gain enlightenment and return life after life until every person has achieved enlightenment. They believe this because: