Karma certainly exists. You live and every action is a seed that grows until it "bears fruit" that fruit is the karma, or "harvest" as Jesus put it. Billion got it part right...our minds harbor the effects of our actions, but our actions also effect others, and how they effect others, determines the consequence "Karma" that comes back to you from those people.
You reap what you sow, so to speak. :D
*Sends out positive vibes* :hippy:
Love,