What is merit?
Merit (Punna) is something done that which purifies and cleanses the mind. It has the power of purifying the mind of the three evil roots mainly greed, hatred and delusion. Thus merit can be looked upon as those actions that improve the quality of the mind. It is the making of merit that ensures one to lead a balanced, righteous and harmonious life.
Another fruit of merit is that “merit opens doors everywhere.” A meritorious man generally finds his way unobstructed, smooth in general. The power of merit will not only be experienced in this life but also and/or in the subsequent life.
Buddha once said merit is one thing that fire nor earthquake can destroy, nor can water drown its effect. In other words, others cannot destroy one's accumulated good merits.
We can acquire merits by performing the ten meritorious deeds below which comprised of 3 big categories: Dana (Generosity), Sila (Morality) and Bhavana (Meditation).
Under Dana comes: charity, sharing of merits and rejoice in others’ merits. This overcomes jealousy and selfishness.
Under Sila comes: Morality, paying respect and providing service in helping others. This overcomes attachment and anger.
Under Bhavana comes: Meditation, listening to dhamma, teaching the dhamma and straightening one’s view. This overcomes delusion and wrong views.
Read more: http://www.suttas.com/10-meritorious-deeds.html