Karma is a word everyone knows, yet few in the West understand what it means. Westerners too often think it means "fate" or is some kind of cosmic justice system. This is not a Buddhist understanding of karma, however.
Karma is a Sanskrit word that means "action." Sometimes you might see the Pali spelling, kamma, which means the same thing. In Buddhism, karma has a more specific meaning, which is volitional or willful action. Things we choose to do or say or think set karma into motion. The law of karma is a law of cause and effect.
Sometimes Westerners use the word karma to mean the result of karma. For example, someone might say John lost his job because "that's his karma." However, as Buddhists use the word, karma is the action, not the result. The effects of karma are spoken of as the "fruits" or the "result" of karma.
Teachings on the laws of karma originated in Hinduism, but Buddhists understand karma somewhat differently from Hindus. (The About.com Guide to Hinduism, Subhamoy Das, here explains karma from the Hindu perspective, if you want to compare.) The historical Buddha lived 26 centuries ago in what are now Nepal and India, and on his quest for enlightenment he sought out Hindu teachers. However, the Buddha took what he learned from his teachers in some very new and different directions.
The Liberating Potential of Karma
Theravada Buddhist teacher Thanissaro Bhikkhu explains some of these differences in this illuminating essay on karma. In the Buddha's day, most religions of India taught that karma operated in a simple straight line -- past actions influence the present; present actions influence the future. But to Buddhists, karma is non-linear and complex. Karma, the Ven. Thanissaro Bhikku says, "acts in multiple feedback loops, with the present moment being shaped both by past and by present actions; present actions shape not only the future but also the present."
Thus, in Buddhism, although the past has some influence on the present, the present also is shaped by the actions of the present. Thanissaro Bhikkhu explained in What the Buddha Taught (Grove Press, 1959, 1974) why this is significant:
"...instead of promoting resigned powerlessness, the early Buddhist notion of karma focused on the liberating potential of what the mind is doing with every moment. Who you are — what you come from — is not anywhere near as important as the mind's motives for what it is doing right now. Even though the past may account for many of the inequalities we see in life, our measure as human beings is not the hand we've been dealt, for that hand can change at any moment. We take our own measure by how well we play the hand we've got."
What You Do Is What Happens to You
When we seem stuck in old, destructive patterns, it may not be the karma of the past that's causing us to be stuck. If we're stuck, it's more likely that we're re-creating the same old patterns with our present thoughts and attitudes. To change our karma, and change our lives, we have to change our minds. Zen teacher John Daido Loori said, "Cause and effect are one thing. And what is that one thing? You. That’s why what you do and what happens to you are the same thing."
Certainly the karma of the past impacts your present life, but change is always possible.
No Judge, No Justice
Buddhism teaches that there are other forces beside karma that shape our lives. These include natural forces like the changing seasons and gravity. When a natural disaster like an earthquake strikes a community, this is not some kind of collective karmic punishment. It's an unfortunate event that requires a compassionate response, not judgment.
Some people have a hard time understanding karma is created by our own actions. They want to believe there is some kind of mysterious cosmic force Out There somewhere, directing karma, rewarding good people and punishing bad people. Some religions may teach that, but not Buddhism. Buddhist scholar Walpola Rahula said,"The theory of karma should not be confused with so-called 'moral justice' or 'reward and punishment'. The idea of moral justice, or reward and punishment, arises out of the conception of a supreme being, a God, who sits in judgment, who is a law-giver and who decides what is right and wrong. The term 'justice' is ambiguous and dangerous, and in its name more harm than good is done to humanity. The theory of karma is the theory of cause and effect, of action and reaction; it is a natural law, which has nothing to do with the idea of justice or reward and punishment."
The Good, the Bad, and the Karma
Sometimes people talk about "good" and "bad" (or "evil") karma. Buddhist understanding of "good" and "evil" is somewhat different from the way Westerners usually understand these terms. To see the Buddhist perspective, it's useful to substitute the words "wholesome" and "unwholesome" for "good" and "evil." Wholesome actions spring from selfless compassion, loving kindness, and wisdom. Unwholesome actions spring from greed, hate, and ignorance.
Karma and Rebirth
The way most people understand reincarnation is that a soul, or some autonomous essence of self, survives death and is reborn into a new body. In that case, it's easy to imagine the karma of a past life sticking to that self and being carried over to a new life. But Buddhist teachings are very different.
The Buddha taught a doctrine called anatman, or anatta -- no soul, or no self. According to this doctrine, there is no "self" in the sense of a permanent, integral, autonomous being within an individual existence. What we think of as our self, our personality and ego, are temporary creations that do not survive death.
In light of this doctrine -- what is it that is reborn? And where does karma fit in?
A full discussion of this teaching would, I'm afraid, take several more essays explaining the Buddhist teachings of self, life and death. In brief, the karma of one life carries itself forward and results in a new life. See this short essay on karma and rebirth, for a little more explanation.