In both Hinduism and Buddhism, every action has consequences. When a pebble falls into a pool, it produces rings that spread throughout and effect the entire pool. The smallest course of action can have monumental effects, even a butterfly fluttering its wings can produce a typhoon, under the right conditions.
In the same way, our actions cause cosmic vibrations that affect not only this life but our lives to come. What we do not learn in this life must be learned in the next. Any harm we may cause in this life will come back to us in the next. The universe is relentless. It will not let us get away with anything, and it's always trying to teach us a lesson.
At the same time, good things we do affect future lives as well. It is said that when the Buddha had his great moment of insight, he saw how all his past lives had prepared him for that moment. He understood how they were connected. All at once, he understood the great force of karma at work, propelling him to come to understand the Middle Way of the Four Noble Truths. With this realization, karma had done its work. He was now complete.
And that, according to the teachings, is what karma does. It makes us complete, driving us forever, if need be, until we come to understanand what we are. And with that understanding, we also come to know who we are. In this grand scheme of things, it is not that we wrestle with God. It is that God wrestles with us and says, in reverse of the words of Genesis 32, "I will not let you go until I bless you!"
Karma is not punishment or retribution but simply an extended expression or consequence of natural acts. Karma means "deed" or "act" and more broadly names the universal principle of cause and effect, action and reaction, that governs all life. The effects experienced are also able to be mitigated by actions and are not necessarily fated. That is to say, a particular action now is not binding to some particular, pre-determined future experience or reaction; it is not a simple, one-to-one correspondence of reward or punishment. Karma is not fate, for humans act with free will creating their own destiny. Karma refers to the totality of our actions in this life as well as past lives; which help to determine our future.