We should begin this answer by considering the definition of a  saint.  Because we are accustomed to thinking of sainthood as something attainable to only a select few, it is difficult for us to realize that we, in fact, are part of the Communion of Saints.  A saint is a good person; not a perfect person but a good person.  They strive to choose what is good; to be holy.  Just as Jesus called his disciples to holiness, we too are all called to be holy – to be saints.
 
The doctrine of the communion of saints flows from our belief that Christians are united as one family in the Spirit of Jesus Christ.  We are united in a bond of love and dependence on one another.  Those of us still living to look to our Christian sisters and brothers for support through both prayer and good works.  We believe that some who have gone before us in faith are in a state of purification (purgatory) and rely on our prayers. Others are experiencing their eternal reward, union with God (heaven), and still, care about and can pray for the living and those in purgatory.  Together, we are the Communion of Saints.
 
Our very first E-Card question was “Why do Catholics pray to saints?”  Looking back, it may have been a good idea for the question of Communion of Saints to precede that one.   When our starting point is an understanding of what it means to be a saint, followed by acknowledging our belief in eternal life and our eternal connectedness, it is understandable that we would ask both the living and those who have gone before us in faith for prayerful support. Originally the word “saint” was synonymous with “martyr,” i.e. someone who witnesses faith in Christ even to death. After a martyr’s death, local Christians endeavored to bury the body in a tomb that would be accessible to the faithful.  On the anniversary of the martyr’s death, Christians would gather to pray and celebrate the Eucharist “in memory of those athletes who have gone before, and to train and make ready those who are to come hereafter” ( Mart. Pol. second century).  Eventually, the memorial celebration of the martyrs occurred in local churches that did not have tombs.  By the fifth century, there was already a feast of “all saints” in the East on the Friday of Easter week.  By the eighth century, the church of “St. Mary to the Martyrs” in Rome seems to have celebrated a similar feast.  In the ninth century, Pope Gregory IV changed the date of the feast to November 1.  From the beginning, those who had endured torture for the faith but had not died (“confessors”) were treated with great respect.  Eventually, Christians who led heroic gospel-inspired lives were often acclaimed after their death as a saint by the local church.  The theology and the celebration of the feast emphasize the bond between those Christians already with God and those still on earth.  The feast points to our ultimate goal – to be with God.

Quote from Encyclopedia of Catholicism