Jesus selected His “dream team,” twelve men who were to be His special followers. He had more than twelve (there is a group referred to as the Seventy), but the Twelve had a special master-pupil relationship with Him.
These were the disciples, and the New Testament sometimes refers to them simply as “the Twelve.” A disciple is a pupil, one who learns.
After Jesus’ death and resurrection, the disciples (minus the traitor Judas Iscariot, who killed himself after betraying Jesus) were always referred to as apostles—that is, ambassadors on someone else’s behalf (Jesus’, that is).
While Jesus was on earth He used the title to distinguish the Twelve from His other followers: “He called His disciples to Himself; and from them He chose twelve whom He also named apostles” (Luke 6:13).
The book of Acts, which follows the four Gospels, is sometimes called the Acts of the Apostles. But in fact, Acts doesn’t tell the doings of all twelve apostles. It focuses on a select few, like Peter, and even on some non-apostles (like the martyr Stephen and the deacon Philip).
And the chief player in Acts is not one of the Twelve at all, but the great missionary and teacher Paul. As far as we know, Paul, unlike the Twelve, had never known Jesus personally—at least, not until the risen Jesus appeared to Paul as he was on a Christian-persecuting mission.
In his many letters that make up a huge part of the New Testament, Paul frequently referred to himself as “an apostle.”
In terms of energy, enthusiasm, travel, and influence, none of the original Twelve held a candle to the apostle Paul. See 996 (twelve).