The law of karma is a special example of the law of cause and effect, which states that our physical, verbal, and mental actions are causes, and our experiences are their effects.
The word karma means 'action' and refers mainly to our physical, verbal and mental actions. The actions we take leave traces or impressions in our very subtle mind that, over time, produce their corresponding results.
Our mind is comparable to a sowing field, and the actions that we commit, to the seeds that are planted in it. Virtuous actions are the seeds of our future happiness, and harmful actions are the seeds of our suffering.
These seeds remain hidden in our mind until they produce their effect, when the necessary conditions for their germination are met. Furthermore, from the time the original action is performed until its consequences mature, several lifetimes may elapse.
As a result of our actions or karma, we are reborn in this impure and polluted world and have problems and difficulties incessantly. Our actions are impure because our mind is polluted by the internal poison of self-clinging. This is the main reason why we experience suffering.
For more information, see The Joyful Path of Good Fortune .