"Karma's only a bitch if you are."
Action. Reaction. Everything that you do initiates a chain of cause and effect, a twisting path of action and reaction extends infinitely out in to our future and rebounds indefinitely into our past. But this infinite chain is circular; what you send out returns.
In the East, karma gives us a guide to this principle. It comes in many forms, practiced by many religions, but at its heart is the notion that what you do, good, ill, neutral, will come back upon you.
It is not a revenge, nor a cosmic punishment; it is more like a ledger, a cosmic balance sheet. You can not do something without some effect rebounding back to you.
In the West, karma is not an explicit part of our philosophy, but it is inherent in the golden rule; do only to others what you would have them do to you.
Karma can guide you. Not by fear of punishment, but by acceptance of balance.