The General Characteristics of Karma.

From nonvirtues come all sufferings,
And likewise, all miserable realms.
From virtues come all happy realms
And the joys in all rebirths.

Like a poison that has been ingested
The commission of even a small sin
Creates in your lives hereafter
Great fear and a terrible downfall.

As when grain ripens in a bounty
Even the creation of small merit
Leads in lives hereafter to great happiness
And will be immensely meaningful as well.

Just as the shadows of birds who dwell.
In the sky move along with them
Beings are followed by
What right and what wrong they have done

Just as when those with few provisions
Set out on the road and travel in suffering,
Living beings who have not cultivated good karma
Travel to the miserable realms.

Just as when those who have prepared many provisions
Set out on the road and travel in happiness,
Living beings who have cultivated good karma
Travel to the happy realms.

Do no scorn even the thiniest sin
Thinking that it will do no harm;
It is through the accumulation of drops of water
That a great vessel gradually fills.

Do not thin that the commission
Of even a tiny sin will not pursue you
Just as a large vessel is filled
By falling drops of water,
So too is a fool filled up with sins
Accumulated a little at a time.

Do not think that the cultivation
Of even a tiny virtue will not pursue you.
Just as a large pot is filled
By falling drops of water
So too are the steadfast filled
By virtues accumulated a little at a time.

By accostiming themselves to virtuous and nonvirtous karma
Humans become habituated to these actions.
Though you may ignore such matters,
In other lives you will experience their effects, 
like a dream.

Those who do not train in generosity, 
ethical discipline, and the like
May have good family lineages, good bodies, and health,
And may have great power and enormous wealth
But they will not find happiness in future lives.

As for those whose family lineage and such are inferior but
who are not attached to sin
And who have qualities such as generosity and ethical discipline,
Their happiness in future lives will grow

As surely as the monsoons of summer fill up the ocean.
Once you have become certain that virtuous and nonvirtuous karma
Give rise to happiness and suffering in lives beyond,
Eliminate sins and make effort at virtuous actions.
Without faith, do as you will.

The brahmins say that virtue and sin
May transfer to others - like giving and receiving a gift.
You (Oh Buddha) thaught that what one has done does not perish
And that one does not meet with the effects of what one has not done.

Further, once you have committed action, you will experience its effect;
And you will not experience the effects of what others have done.

Even in one hundred eons
Karma does not perish.
When the circumstances and the time arrive
Beings surely feel its effects

From : The Great Treatise on the Stages of the Path to Enlightenment
LAM RIM CHEN MO
TSONG-KHA-PA