In the broad sense, a saint is anyone living in a state of grace or who has died in the grace of God. Such saints make up what's called the Communion of Saints, which are the triumphant in Heaven, the faithful striving for holiness on earth and the Holy Souls being purified in Purgatory. In this sense St. Paul in Romans 15:25 says, "I shall go to Jerusalem, to minister unto the saints."
In a narrower sense the saints are all the citizens of Heaven. In the narrowest sense, however, a saint is someone the Catholic Church has declared to be in Heaven by canonizing them. The Church attests that a canonized saint lived a life of heroic virtue, died in God's grace and is in Heaven. In order to be declared blessed, or beatified, a saint must have worked one Church-verified miracle after death and one more prior to being canonized. The miracles attest to the fact that the person is in Heaven and that God wants his holiness recognized. A person's enrollment in the canon of saints is called a dogmatic fact and is infallible.
When the Church canonizes a person, it authorizes public veneration of him and showcases his life of virtue to be imitated. It also recommends him to the faithful as an intercessor with God. Honoring or venerating the saint is not worshipping him as God, but rather seeing him as God's special handiwork. Praying to a saint, likewise, is simply asking for his prayers. James 5:16 reads, "[P]ray one for another, that you may be saved. For the continual prayer of a just man avails much." The saints in Heaven are completely just, perfectly willing to assist those on earth, and raptured in prayer.
It's Church custom to name children after a saint at baptism and for confirmation. Patron saints are chosen not only for particular persons but also for places, communities and organizations. The Roman Martyrology lists all the canonized saints.
Become familiar with the friends of God in season one of Church Militant's Premium show The One True Faith—Communion of Saints.