Saints are people who were open to God in a special way during their lives. Because they were declared a saint (canonised) by the Church after their death, it is clear to all that they are in heaven, and pray for us there. They are an example for all believers.
The Church has instituted a specific procedure for canonisation. After a person’s death, he or she first has to be declared a ‘Servant of God’, then ‘Venerable’, then ‘Blessed’, and finally ‘Saint’. Everyone can become holy by living in a close relationship with Jesus, even though not all such people are officially declared saints.