On the last Sunday of October in the Vetus Ordo the Church celebrates the feast of Christ the King. Our Lord reigns over the Church, of which on earth the Pope is only His vicar. In both forms on November 1, we celebrate the Church Triumphant and all of its members, known and unknown to the living faithful. The next day is the Commemoration of the Faithful Departed, or All Souls Day as we pray for those deceased Christians (and also those whose faith is known only to God).
November is a month dedicated to the Holy Souls in Purgatory, the Church Suffering. We must acknowledge them and pray for them as they are a part of the Mystical Body of Christ, and part of being in communion, in unity in Christ's Body is praying for its parts. On Earth we pray for the Pope, Bishops, clergy, and the living faithful. We pray in union with the angels and saints, and we ask for their intercession (we can't pray for people in Heaven, of course). It is particularly important to pray for the souls in Purgatory, for they are suffering, and it is beyond our earthly comprehension the triumph of the beatific vision, when one more saint is welcomed to the eternal supper of the Lamb. One more saint to pray for us, so that we might be there one day too.
In certain places in the United States, it was the practice prior to the liturgical changes for one priest to say the daily Mass for the dead, since it's shorter (no Psalm 42) and another pray the Mass of the day. It isn't the best way to approach liturgy, but now we forget by-and-large to pray for the dead. An explanation of that requires another post.
There is a plenary indulgence that can be earned during the Octave of All Saints, November 1-8 (sorry, school got in the way of posting earlier...) for praying for the faithful departed in a cemetery. This is earned under the usual conditions of praying for the Pope's intentions and receiving Communion and Confession, and being without attachment to sin. Otherwise, and at all other times, it's a partial indulgence.
This prayer is often used: Requiem aeternam dona ei(s), Domine, et lux perpetua luceat ei(s). Requiescat (Requiescant) in pace. Fidelium animae per misericordiam Dei requiescant in pace. Amen.
Eternal rest grant unto him/her (them), and let perpetual light shine upon him/her (them). Rest in peace. May the souls of the faithful departed, through the mercy of God, rest in peace. Amen.
It is a good custom to add the second half of the prayer to the blessing before meals. That means you pray for the dead at least thrice daily.