Karma literally means action or 'to do.' It is active and dynamic. Many people mistake karma to mean 'predestination' as the Calvinists believe. Yet, this is not quite accurate. For example, you may have heard people say that someone is rich because it is his karma. But how did this particular person become rich? Did he inherit wealth or did he start a business which became prosperous? Whether he inherited or started his own business, somebody planned and worked to bring all that planning into fruition. The action someone takes to start a business is conscious and deliberate. This conscious action is Karma-Vipaka. So, karma and vipaka are closer to the third law of Newton than the belief of predestination by the Calvinists. The third law of Newton states that for every action , there is an equal and opposite reaction. Therefore, success follows the hard work of consciously running a business well.
Rebirth in the Buddhist sense is determined by karma. Above, I described how karma is a very active ideology. Therefore, what we are in this life determines what we will be in the next life. Buddhists believe that a person is made up of
"thoughts, feelings and perceptions interacting with the body in a dynamic and constantly changing way." (Buddhanet Glossary of Buddhist terms - link below)
Therefore, Buddhists don't believe in the immortality of the soul as Christians do: rather, they believe that human beings are the sum total of these mental energies, which can be transmitted to a different host body in the next life.