The Blessed Buddha once said:
Beings are owners of their actions (kamma = karma), inheritor of
their actions, are created by their actions, linked to their actions,
their actions produce their destiny. Whatever actions they do;
good as evil, the resulting reaction & effect will be only theirs!
There is one who kills living beings, steal what belongs to others,
commits adultery with other's partners; speaks lies, uses divisive
and aggressive speech, prattle empty gossip; is covetous, envious,

jealous, wicked-minded of evil views. Such one is creeping in all
actions both bodily, verbal, and mental. Hidden & secret are such
one's actions, words, and thoughts, of ulterior & concealed ways.
But I tell you: Whoever pursues hidden ways and objects will have
to expect one of these two results: Either the torture of hell, or
birth among the creeping animals. Thus it is with all rebirth of any
being: They will be reborn according to their actions (kamma)...
When reborn, they will experience the exact result of their actions.
Therefore I declare: Beings are owners of their actions (kamma),
inheritor of their actions, are created by their actions, linked to
their actions, as a shadow that never leaves! Their actions produce
and condition their future destiny. Whatever action they perform;
good as evil, the resulting reaction & effects will be only theirs!

Kamma causes & creates rebirth! A shadow that never leaves!

Source:
Numerical Discourses of the Buddha. Anguttara Nikaya AN 10:205
http://What-Buddha-Said.net/Canon/Sutta/AN/Index.Numerical.htm
 

For details on the mechanics of Kamma = Intentional Action see:
http://What-Buddha-Said.net/drops/II/Kamma_is_intention.htm
http://What-Buddha-Said.net/drops/II/Buddha_on_Kamma.htm
http://What-Buddha-Said.net/drops/III/Inevitable_Consequences.htm
http://What-Buddha-Said.net/drops/II/Good_Action_dilutes_Evil_Kamma.htm
http://What-Buddha-Said.net/drops/II/Kamma_leading_to_short_&_long_life.htm
http://What-Buddha-Said.net/drops/II/Kamma_leading_to_Health_&_Sickness.htm
http://What-Buddha-Said.net/drops/II/Kamma_leading_to_Low_or_High_Birth.htm
http://What-Buddha-Said.net/drops/II/Kamma_leading_to_Wealth_or_Poverty.htm
http://What-Buddha-Said.net/drops/II/Kamma_leading_to_Beauty_&_Ugliness.htm
http://What-Buddha-Said.net/drops/II/Effect_of_Action_(kamma)_is_Delayed.htm
http://What-Buddha-Said.net/drops/II/Kamma_leading_to_Power_or_Disrespect.htm
http://What-Buddha-Said.net/drops/II/Evil_Kamma_enhances_other_Evil_Kamma.htm
http://What-Buddha-Said.net/drops/II/Good_Action_enhances_other_Good_Kamma.htm
http://What-Buddha-Said.net/drops/II/Kamma_leading_to_Stupidity_or_Intelligence.htm