The doctinre of the communion of saints is based on the idea that members of the body of the Christ, i.e. the Church, are never cut off form the body; not even in death. Accordingly,we may ask the dead to pray for us, as we ask the living to pray for us, (because no one who lives in Christ is ever truly dead in the literal sense of the word, but to make things simpler I used the word dead, to desribe those who have left this life). We are recommended to pray for all members of the body of Christ. This includes those we know on this earth, and those who have left this earth; particularly the souls in purgatory whose time in purgatory may be reduced by our intercession, and who can intercede for our requests, when they reach heaven. There is a more detailed thread on this forum about prayers for those in prugatory.
Some Protestants disagree with the Catholic practice of asking saints or those in heaven to intercede our causes. This is largely based on their erroneous belief that asking the saints to intercede some how places them equal to the role of Christ. Catholics equate no one with Christ. The idea of saints interceding is that the saints pray to God for us too, just like our friends on earth do. Much in the Old Testmaent and New Testament talks about the power that the righteous have in their prayers, and the fact that some one is in heaven, is an unassailable account of their righteousness.
Some Protestants go as far to criticize the practice of praying for to saints as equal to necromancy. However, Catholcis are not evoking spirits to leave heaven, come to the earth and do their bidding or tell them the future, but merely asking those in heaven to pray to God for us.
Catholics do not believe that once some one dies that they are inactive in the life of the Church. Revelation speaks of the souls of martyrs beign in the altar and calling for retribution; a pasage which Catholic theologians use to demonstrate that the dead don’t stop concernign themselves with justice. Catholics believe that the sprits of the dead intercede for us in heaven through prayers, and participate in the sacrament of the mass with us.