... it extends beyond and before a single lifetime. So the full karmic effects are not necessarily experienced during this lifetime and the reason why shit happens can also be due to the karma from previous life times.
In some sense karma is part of a believe system. I do believe that it is real, but somethings aren't necessarily explained. In some cases things have happened to me which I had done unto other, but they are not that frequent so you'll end up discussing statistical anomalies. Also the amount of actions per day are immense
For a more detailed explanation:
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SOME COMMON EXPLANATIONS OF KARMA
While there are a number of explanations of how karma functions, there does seem to be a consensus, and it is these generalized explanations that we will consider.
"Do unto others as you would have others do unto you." This is often called the Golden Rule, and forms the cornerstone of every major religion.
The law of Karma, is also known as the law of Cause and Effect. In the material world, it has its equivalent, and this is enshrined in Newton's third law of motion: "To every action, there is an equal and opposite reaction".
http://www.marcus-clark.com/path/karma1.html