Karma generally means the cause and effect of our thoughts and actions. But the word karma has several connotations. It can mean an action, its reaction, or the whole system of action-reaction known as the law of karma.
"Do unto others as you would have them do unto you" and "what goes around, comes around" are common ways of talking about karma. Basically, our good or bad actions determine our good or bad future. Our quality of life, in terms of such things as health, wealth, intelligence, and physical appearance, is the result of our previous karma, actions.
We can change our karma from good to bad (or bad to good) by changing our activities. But any karma is bad from the spiritual point of view. In our natural, free condition, the spirit self, atma, isn't meant to live under strict karmic laws. Good and bad karma is what forces us to endure repeated birth and death. This upsetting process forces us into a succession of "good" or "bad" temporary bodies, in an environment characterized by constant upheaval—the material world—that's alien to our eternal spiritual nature. But karma is not something we have to be stuck with forever.