Karma

Karma means "(the result of) action", generally taken as a term that comprises the entire cycle of cause and effect. Karma is a sum of all that an individual has done, is currently doing and will do. Individuals go through certain processes and accompanying experiences throughout their lives which they have chosen, and those would be based on the results of their own creations: "karma". Karma is not about retribution, vengeance, punishment or reward. Karma simply deals with what is. The effects of all deeds actively create past, present and future experiences, thus making one responsible for one's own life, and the pain and joy brought to others. In religions that incorporate reincarnation, karma extends through one's present life and all past and future lives as well.
Karma is simply the golden rule: what you give out is what you receive - either in the same or in similar form. You reap what you sow - your actions create that which you do live out now, whether this relates to a past/future life situation or to the present date.
Karma is not fate, for man acts with free will creating his own destiny.
Not all karma rebounds immediately. Some accumulate and return unexpectedly in this or other births.
Buddhists believe that the actions of beings will affect their own future, and because of this there are no private actions: all actions have a consequence. The emphasis of karma in Buddhism is on cause, not on effect.
If an individual sows goodness, he or she will reap goodness; if one sows evil, he or she will reap evil. Karma refers to the totality of mankind's actions and their concomitant reactions in current and previous lives, all of which determine the future. However, many karmas do not have an immediate effect; some accumulate and return unexpectedly in an individual's later lives. The conquest of karma is believed to lie in intelligent action and dispassionate reaction.
Unkindness yields spoiled fruits, and good deeds bring forth sweet fruits. As one acts, so does he become: one becomes virtuous by virtuous action, and evil by evil action.

For more info that includes a little to much reading for me or things I'm not concerned with here, you can go to Wikipedia, which is where I got all of the above information.
http://en.wikipedia.org/wiki/Karma