What is karma and how does it manifest?
Karma means "action" in Sanskrit. According to the teachings of Buddha, the circumstances of one's birth, both fortunate and unfortunate, are the direct result of past life karma. Every virtuous and non-virtuous word, thought, or action creates an imprint which at some point will be reflected back as either positive or negative karma. It is the karma of action that holds more consequence than the karma of thought. Also, it is one's intention, or mental attitude, that largely determines the weight of the karma. For example, one's motivation in helping someone would be more pure if it arose from a place of compassion instead of duty. Likewise, a premeditated harmful action would bear more karmic results than harmful action that was impulsive. Thus, the purer the motivation, the stronger the manifestation of the karma.
How Karma Unfolds
The mind, although formless, is a separate entity from the body. After the body passes away at death, the mind does not cease, but passes into a deeper subconscious state. According to The Tibetan Book of the Dead, this state is called the bardo state, meaning intermediate state. In this place, past karma, both positive and negative, arises in the mind-essence. By chance, one ripens and rebirth follows to fulfill that karma. It is a misconception to think that karma has to follow a linear path; karma can ripen many lifetimes after the particular life in which it was created.
Ceasing the Effects of Negative Karma
Many times difficulties seem to arise out of nowhere. Our usual response is to complain or blame, which is a negative response to the already negative karma manifesting from our past. That kind of response creates even more negative karma. A more conscious reaction when we are having difficulties, is to accept the blame, as it is our actions from the past that created the present difficulty. We should not have harsh thoughts or actions towards ourselves or anyone else regarding the matter. We create positive karma when we respond positively to the negative. Many times this can also clear the effect of the particular negative karma. And when we are are experiencing the effects of positive karma - when things are going well in our lives - we should use the time to further develop equanimity, so that when faced with difficult challenges again, we have the ablitiy to respond with more acceptance and patience. We have this potential at every moment of our lives.
Ending Karma and Rebirth
The most positive karma comes from the motivation to relieve all beings from suffering. The purest way to do this, is to to liberate ourselves from the bondage of the ego, as our liberation, (or salvation in the Christian religion), helps to free others from the bondage of their ego. It is our egos that separates us from One Mind, God-Consciousness, Universal Love. etc. Once we are liberated, we are free from the effects of karma and rebirth. However, a liberated being might choose, out of deep compassion, to be reborn to help others purify their minds and open their hearts. In Buddhism, these beings are called Bodhisattvas (bode-e-saht'-vas). (Some people think of Christ as one.) It is also thought that Bodhisattvas can disguise themselves to become teachers for us, so we never know if our spouse, friend, or neighbor is such a being. Perhaps it was not "our child" that died so young from illness, but a great Bodhisattva taking the body of a child: To be born to us, to be loved by us, and to be grieved by us - all for the purpose of opening our hearts more deeply.
Good Works as Positive Karma
If we don't have liberation as a goal, we should at least do good works to help others from their immediate suffering. Every good action, thought or word creates positive karma, so even a kind smile or uplifting thought towards someone is beneficial. Being kind, as a means to create positive karma for ourselves, would essentially seem be a selfish act, and ultimately it is. Therefore, we dedicate the positive karma that we have created to other beings - i.e., we give it away - a very loving act! By giving it away we don't reinforce our egos with ideas of "good karmic medals", so to speak. In the beginning, we may not be able to do this very sincerely, but as we practice daily, our compassion becomes more pure and authentic.