Karma could be described as your accounting system for good and bad deeds.  It is the net result, the consolidation of all your actions in this and all previous lives. It cannot be cheated; it is the way of the universe to keep track of your past actions. Every time you are reborn as a human being, it means that you had enough karma to get reborn as a human; when your karma drops too low because of bad actions in your last life, you will not be reborn as a human being but as an animal. Humans are the most advanced species on Planet Earth at the moment, as we are the only ones with a special gift to use our intellect to for a higher understanding and the ability to reach the goal of self realization.
Karma is guided by the thoughts you hold in your head when you are doing good (or bad) deeds.  For instance when you help somebody by doing dishes, it’s only good karma when you are actually willing and happy to do it.  While performing the act, you should be completely at peace. No points are given for work done against conscious will!