The purpose of understanding and believing in karma is to prevent future suffering and to establish the basic foundation for the path to liberation and enlightenment.  Generally, karma means 'action'. From non-virtuous actions comes suffering and from virtuous actions comes happiness: if we believe this, we believe in karma.  Buddha gave extensive teachings that prove the truth of this statement, and many different examples that show the special connection between the actions of our former lives and our experiences in this life, some of which are explained in Joyful Path of Good Fortune.