Here is a redo of the assumptions Warpus. I will work on the rest. this is very simplified as you will see if you read the links.
Some of the basic assumptions underlying the idea of karma
• The fundamental nature of existence is
Emptiness or selflessness (Buddhist)or
Brahman (Hindu)
• Creation (the physical universe) is not Real, but transitory and impermanent. Everything in it comes and goes, appears and disappears and changes over time. People are born, grow old die, and decompose. Steel rusts, mountains are built and erode, chemical mix and change, suns burn and then don’t. All things change. No thing within the physical universe is permanent.
• Within Creation, the physical universe, things appear to be separate and distinctive from one another. I see myself as a separate entity from the people and things around me.
• Souls exist and inhabit life forms. In those forms they experience life as that entity. Upon the death of a life form, its soul transmigrates into another life form.
For many those assumptions will make no rational sense, but if you are going to talk about karma, you need to be aware that they are lurking all around.
EDIT: Bolded sentences are from my original post.
Hindu and Buddhist views on karma are not the same so any single explanation will be incomplete.
Within creation the soul experiences itself as an individual, separate entity. Underlying Hinduism & Buddhism is the idea that Brahman or Emptiness is all that truly exists on a permanent basis. The physical universe, though, appears to be divided into many distinct and separate entities (from quantum particles up through life forms, planets and stars). The soul experiences this separateness and its own individuality.
The soul accumulates the impressions of experiences it has as that entity. If a soul is experiencing life as a person, then the experiences it has as a person are “impressed” upon the soul, attached to or associated with it. Millions of experiences are thus accumulated. Some associations are stronger and more meaningful than others; some are good and some are not so good.
This binding of the soul with experiences of separateness continues life after life. Many of the experiences reinforce the notion of individuality and separateness from the other people and things of the physical universe. In life after life the collection of experiences continue to accumulate.
The various experiences of a soul reinforce its identity with its current life form. The greater the accumulation of experiences a soul has as a particular life form, the more it identifies with that life form.
The purpose of the soul is to remove all those impressions of experience so that it can experience its unfettered self and to do so the soul must balance each experience with its opposite. Since all that is actually Real is Brahman/Emptiness, the real goal of the soul is to experience that fundamental state. To do so it has to rid itself of the false impressions/experiences of separation that it has accumulated as part of the physical universe. When it does so, it will experience itself as Brahman or as the Buddhist Emptiness state. The process of ridding itself of accumulated experiences is one of balancing out opposite types, experiencing all that is possible so in the end all are gone. In both Hinduism and Buddhism selflessness can speed the process because it undermines the sense of separate identity that has accumulated around the soul over time.
Karma is the universal law that binds and unbinds our actions to our souls. Karma is the process that “manages” the accumulation of experiences and the shedding of those that are ready to go. It looks back at where you have been and and brings you to your present, based on your past actions. One’s past experiences create one’s present and then you choose (or not) how to move forward. Accumulations of selfish or unkind actions may well lead additional situations where one is tempted to act selfishly and further bind one to such experiences in the future. Kindness and selflessness can have the effect of speeding up the process of shedding experiences that keep you further from Brahman or Emptiness. Then once the soul is completely free from its experiences as a separate and isolated entity, it will experience Brahman or Buddhist Enlightenment. The Illusion of individuality and separateness that is the physical universe disappears and one experiences Truth.
I hope that this is clearer. Keep in mind that this is a very simplistic description of two very complicated religions, but I've tried to put karma in its proper context.