The Chaplet of the Divine Mercy is a Roman Catholic devotion based on the visions of Polish nun and canonized saint, Sister Mary Faustina Kowalska (1905-1938), known as "the Apostle of Mercy." The chaplet is often said as a rosary-based prayer with the same set of rosary beads used for reciting the Holy Rosary or the Chaplet of Holy Wounds. However, the chaplet may also be said without beads, usually by counting prayers on the fingertips.
According to Sister Faustina's visions, written in her diary, the chaplet's prayers for mercy are threefold: to obtain mercy, to trust in Christ's mercy, and to show mercy to others. Still, according to Sister Faustina's visions, Jesus Christ promised that all who recite this chaplet at the hour of death or in the presence of the dying will receive great mercy. Jesus said, "....When they say this Chaplet in the presence of the dying, I will stand between My Father and the dying not as the just judge but as the Merciful Savior." Jesus also promised that anything can be obtained with this prayer if it is compatible with His will.
For more http://en.wikipedia.org/wiki/Chaplet_of_Divine_Mercy