In prayer, you consider yourself separate from God, and then pray to God. If prayer gets concentrated, it becomes more effective - because the Ultimate Truth of the mind is God. When we pray, we should pray for everyone, just as in the prayer we recite "Loka Samastaa," we pray that everyone should be in peace and prosperity. If the mind becomes concentrated, it goes inwards, it becomes introverted, and touches the Divine. Thus prayer can be so effective. In the same way, when we meditate, the mind becomes concentrated, and gradually goes inwards and loses all the imaginations of the mind, and merges with the Self. Both are effective and both are the same.