Definition: "Karma" is a Sanskrit word which literally means "action." So our karma is our actions. Good actions, bad actions, and neutral actions will give rise to happy results, suffering results, and boring results, respectively.
Misconceptions: Karma does not "rub off." We ourself will eventually experience the results of our own actions.
More information on karma: http://kadampa.org/reference/karma
A lovely, inspiring video about karma: