When the Church prays for the dead she prays for the repose of the Holy Souls in purgatory.
Those in heaven have no need of such prayers (the faithful rely upon theirs, however, and constantly appeal to the saints), while souls in hell cannot benefit from the prayers of the living.
The purpose of prayers for the dead is to help to bring the Holy Souls home to heaven. The Church prays that they will finally rest in peace and offers atonement for the sins they have committed on earth.
November is set aside by the Church to pray for the Holy Souls. Although November 2 is a day especially dedicated to them, the faithful should find time to pray privately for them on every day of the month, with those who died in the war remembered publicly and privately around November 11.
Any Friday in November is a good time to offer a rosary in reparation for the sins of the deceased.
Masses can be offered for relatives and most priests organise lists of people formerly of their parishes to be remembered. Try also to remember those who have no one praying for them, and those who had died young or in particularly tragic circumstances.