A saint (from Latin, sanctus) is one who is holy, that is, set apart for God's service.
Saints in the Bible
In the Holy Scripture, the word saint is used to refer to those who have been set apart for the service of God, consecrated for his purposes. As such, all members of the Church are called saints, regardless of their personal holiness or sinlessness. It is still appropriate to use the term in this way.
Saints in the Church
Aside from the more general use of the word saint to refer to all members of the Church, Holy Tradition also ascribes Saint as a title to particular persons whose lives have shown most clearly what it means to follow Jesus Christ. These saints are popularly glorified (canonized) by the Church, often in the modern era with a formal service to recognize and affirm the veneration of them by the faithful.
Saints are not thought of as either perfect or infallible, and it is only because of the work of Christ in them that the Church praises these people. It is because we see our Lord's countenance reflected most clearly in their faces that we publicly laud them, ask them to pray for us, and encourage one another to follow their examples.