The law of cause and effect, karma, is one of the main teachings of Buddhism. It means that we should always practise virtuous things, since non-virtuous acts will always bring suffering in this life as well as in the next. If we don’t want suffering, we should avoid its cause; if there is no cause, there will be no result, just as if the root of the tree is removed, there will be no fruit. If we want happiness, we must be very careful about the cause of happiness, just as if we want the tree to grow, we must take care of its root. If the root is defective, the tree will not grow.

-- His Holiness Sakya Trizin