Saints in Catholicism
By one common definition of the word, a saint is a person who has been recognized by a Christian denomination and approved for public and private veneration.  Saints, especially those from the Apostolic and Ancient periods of church history, often were martyrs (people who died for their faith or for another person).
In Catholic tradition and law, all saints are associated with miracles, either in this life or thereafter, through miracles that occur when praying to them after their deaths. Catholic and Eastern Orthodox doctrine holds that saints are able to intercede with God on behalf of living persons, or for mankind generally, and that miracles may occur as a result.
Saints in Roman Catholicism are canonized according to a process which first requires election by any among the faithful to the Vatican Sacred Congregation. The election is made by written petition, which, in the case of a recently living person, requires eyewitness testimony to his miracles. In the case of an ancient cause, only written testimony of the elected person's miracles are admissible. For both ancient and recent causes, the petitioner must make a "chronologically arranged report on the life and deeds of the Servant of God, on his virtues or martyrdom, on his reputation of sanctity and of signs." The cause's miracles, martyrdom, and virtues are examined by the presiding Bishop of the Sacred Congregation.  Usually a person is canonized if at least three miracles have been performed by them through lay people asking the deceased to intercede with God on their behalf. These miracles must be thoroughly investigated by church authorities and declared valid before canonization occurs. For Eastern Orthodox Christians, saints are those whom the whole body of church members believe to be saints.
Many martyrs are recognized as saints based upon what Jesus said: the greatest commandments are to love God with your whole heart and soul and to love your neighbor as yourself. He also told His disciples that 'greater love than this no man hath, that a man lay down his life for his friends.'
Among Roman Catholics, the Queen of Saints is the Blessed Virgin Mary, the Mother of Jesus Christ the savior. Her special and filial reverence as the Mother of Christ is commended to all Catholics "[t]o foster the sanctification of the people of God."  Notable Saints include the Twelve Apostles, St. Paul, St. Augustine, St. Patrick, St. Francis of Assisi, St. Theresa of Avila and St. Pius X.
Saints in Protestant Tradition
Because the formal process of canonization described above is a distinctly Catholic rite which requires acceptance of the authority of the Vatican to recognize saints for public veneration, and because many Protestants reject Catholic teaching on sainthood as un-Scriptural, Protestants adhere to a different methodology. Among Lutherans and some other Protestants, the only saints generally accepted are those who are clearly indicated in scripture, such as the Twelve Apostles. Anglicans have canonized saints by action of a church convention or, in the case of the Church of England, by Parliament. Other Protestant bodies prefer to apply the word to any true believer. This accords with the New Testament's usage. Peter referred to all believers as priests and ministers of Christ, and, in addition, as "a holy nation". 
Lutherans and Methodists hold that deceased believers may pray for the living and commend them for their faith and acts and as good examples to follow. Lutheran, Anglican, Methodist and most other Protestant churches prohibit praying to saints, however.
- Canon 1186 
- New Laws for the Causes of Saints (1983)
- Canon 1186 
- What Are Christian Saints According to the Bible?
- See for example, First Epistle of Peter, 2:9
Beatification and Canonization, http://www.newadvent.org/cathen/02364b.htm