These were the 12 followers of Christ, called by him at the beginning of his ministry. They are named in the Gospel of Saint Matthew (Matthew 10: 1-4).
The first to be called were Andrew and his brother Simon (later called Peter), James and John, the sons of Zebedee, all of whom were fishermen, then Matthew, the tax collector, Philip, Bartholomew, Thomas, James (son of Alphaeus), Thaddeus, Simon of Canaan and Judas Iscariot, who betrayed Jesus, and hanged himself. He was replaced by Matthias (Acts of the Apostles 1).
After Christ's death and Resurrection, the apostles were responsible for spreading the Christian faith throughout the world.