As an Indian, I'd like to chip in with the actual meaning of "Karma" - the closest synonym in English is "fulfilling your purpose".
The Indian culture is heavy with assumptions that everyone has some purpose in life. The concept of karma urges you to act upon your purpose in life - intended or perceived. It never promises any rewards or punishments by itself. That is primarily taken care of by the rest of the philosophy in these books - unsurprisingly similar to all the other religions of the world.
By itself, karma has no positive or negative association. In fact, one of the holy texts dissociates your "karma" with the fruits reaped subsequent to it. "Continue fulfilling your purpose in life without expecting any rewards for your actions whatsoever." In other words:
Happiness = Reality - Expectations.
The lower the expectations and 'better' the reality, the greater your happiness. :)