The twelve apostles were the disciples Jesus chose as his closest followers. The Gospels of Mark (chapter 3), Matthew (chapter 10), and Luke (chapter 6) list them, with small variations, as Peter and his brother Andrew; James and John, the sons of Zebedee; Philip; Bartholomew; Matthew; Thomas; James, the son of Alphaeus; Jude, also called Thaddaeus; Simon the Zealot; and Judas Iscariot.
After Judas Iscariot betrayed Jesus and died, the remaining apostles chose Matthias to take his place (Acts 1). The word apostle means one who is sent, and it is sometimes also applied to others, most notably Paul, who became a Christian a few years after Jesus’ death.
Source: Encyclopaedia Britannica: Apostle
Image: The Last Supper by Leonardo da Vinci, public domain, via Wikimedia Commons.