From an essay by Thanissaro Bhikkhu:
The two most important points in the Buddha’s teaching on karma — the two that set it apart from every other version of karma taught in his time — are that karma is intention, and that present experience is shaped by two kinds of intentions: past and present. Past intentions that are ready to ripen establish the range of possibilities that you could experience right now. Your present intentions pick and choose from those possibilities to shape what you actually experience. …
As you meditate, these points alert you to the fact that some things you experience in the present come from past intentions, and some from present intentions; you have to be able to tell the difference between the two if you want to read the results of your present actions.
Found at The Worst Horse.