THE LAW OF KARMA
If man speaks or acts with an evil thought - pain follows as the wheel follows the foot of him that draws the carriage - But if man speaks and acts with a pure thought, happiness follows him like a shadow that never leaves him.
When something wrong or unpleasant is done to us - the natural reaction is retribution or revenge. Think how it becomes a vicious circle - no one is the winner and more hatred and negativity is built up.
Rather if someone does you wrong you should realize that their action has its own reaction, the Law of Harmony (Karma) balances everything out. If we can "turn the other cheek" so to speak, and not feel the need to punish or seed revenge, then we are growing spiritually, and not incurring any bad Karma of our own.
We should never feel G-d or some Higher power punishes us for our deeds. Karma is not crime and punishment, and one good deed doesn't cancel out a bad deed. Perhaps the simplest examples can most easily be understood.
If you sow wheat you reap wheat.
If you put your hand on a hot stove you will reap blisters and pain. There is no punishment involved. The pain and blister is merely the result of your mistake in touching the stove, and presumably you'll have learnt not to make the same mistake again.
Our Karma is simply the total consequence of our acts good/bad, whether intended or performed in ignorance.