Karma in hinduism is cause and effect.
In buddhism karma means cyclical thought pattern.
It's a kind of vice of mind, you use meditation to be aware of some kind of recurrent thoughts and feelings that come to your mind without your conscious will. The karma can be easily spotted in situations when you react in an automatic way.
The objective of buddhism is to train you mind until you have full control upon your karma. Because the natural manifestation of karma is one of the sources of suffering or insatisfaction.
I do not believe in this, I just accept as an ancient decent psychological explanation.