Karma is understood as that which causes the entire cycle of cause and effect. Remember the saying "you reap what you sow." That definition of karma works too.
We move through our lives and sometimes forget that simple saying. I grew up hearing it, and I truly believe it wholeheartedly.
You cannot do wrong by someone, and think for one second it will not come back on you.
You cannot cheat someone, and think you will never get cheated.
It is all a cycle, and you get as good as you give.
Is it hard? Hell yeh it is hard sometimes to turn the other cheek, but when you think about what that person that has wronged you will get - it becomes worth it. One thing that is also gratifying is - it does not usually take that long.