karma comes from the sanskrit word 'karman and simply means action.
Bad karma leads to bad consequences. You have to experiences the fruits of your actions. Creation is said to be reaction, reflection, resound; in other words, whatever you think, say and do comes back to you.
Good karma brings good fruits. Good action brings back good.
Action without desire for the fruits of the action is called selfless activity. Selfless activity can be service, or actions dedicated unto God. This is called nishkama karma; action that brings about no negativity.
So it is better to engage in good karma or selfless karma. Any other karma causes suffering and rebirth. If you die with unfulfilled wishes or with action you have done (such as harm to another person) then you have to come back in a new body and experience the harm or injury you caused to another. Buddhism teaches this also, it is called dependent origination, birth caused by previous actions.