We are not human beings having a spiritual experience.Karma comes from the root kri, "to do". The principal is based on the idea that actions determine your destiny. One becomes good by good action and bad by bad action. Each karma, or action, generates a vibration that continues to vibrate in the mind. These vibrations are subconscious impressions. Like attracts like. Love attracts loving acts, malice attracts malice. The attraction and reaction is non-judgmental, and does not involve the operation of morality or a single divine judgment.
We are spiritual beings having a human experience.
Fate and Karma are very different. Fate means that one's life has been predestined. Karma is exactly the opposite. With Karma, your destiny is completely under your control, be it for good or for bad. The basis by which the two work is something of a paradox, and is suggested as a further area of study.
Karma works hand in hand with reincarnation. Reincarnation means rebirth of the soul into a new body based on the soul's spiritual level, for the purpose of learning certain life lessons. It is a journey of the soul, simply put. The soul makes a choice about the lesson it will learn during a physical incarnation, and the lesson(s) are largely based on the karma built up during past incarnations.