The Karma or sum of good and bad deeds that a person did in one life, will affect the quality of their next life. Buddhist believe that everyone is reborn, therefore they think that Karma is important in once life.
Karma in Buddhism is the force that drives samsara - the cycle of suffering and rebirth for each being. Good, skillful deeds and bad, unskillful actions produce "seeds" in the mind which come to fruition either in this life or in a subsequent rebirth. The avoidance of unwholesome actions and the cultivation of positive actions is called sila (from Sanskrit: "ethical conduct").
In Buddhism, karma specifically refers to those actions (of body, speech, and mind) that spring from mental intent ("cetana"), and which bring about a consequence (or fruit, "phala") or result.