St. Paul is called "the Apostle to the Gentiles". However, he was not one of the original twelve apostles. He referred to himself as an apostle who was "born out of time". Unlike the other apostles, he was chosen by Jesus through a vision. In Chapter 9 of the Acts of the Apostles it states: On his journey, as he was nearing Damascus, a light from the sky suddenly flashed around him. He fell to the ground and heard a voice saying to him, "Saul, Saul, why are you persecuting me?" He said, "Who are you, sir?" The reply came, "I am Jesus, whom you are persecuting. Now get up and go into the city and you will be told what you must do." The men who were traveling with him stood speechless, for they heard the voice but could see no one. Saul got up from the ground, but when he opened his eyes he could see nothing; 3 so they led him by the hand and brought him to Damascus. (Acts, i, 15-26)
Also, see information on St. Paul at: http://www.apostles.com/stpaulbiography.html
The original Twelve apostles were:
After the Judas' betrayal of Jesus and death, Matthias, a long-time follower of Jesus but not one of the 12 apostles, was chosen to replace Judas.
In Chapter 1 of the Acts of the Apostles it states: During those days Peter stood up in the midst of the brothers (there was a group of about one hundred and twenty persons in the one place). He said, "My brothers, the scripture had to be fulfilled which the holy Spirit spoke beforehand through the mouth of David, concerning Judas, who was the guide for those who arrested Jesus. He was numbered among us and was allotted a share in this ministry. He bought a parcel of land with the wages of his iniquity, and falling headlong, he burst open in the middle, and all his insides spilled out. This became known to everyone who lived in Jerusalem, so that the parcel of land was called in their language 'Akeldama,' that is, Field of Blood. For it is written in the Book of Psalms: 'Let his encampment become desolate, and may no one dwell in it.' And: 'May another take his office.'
Therefore, it is necessary that one of the men who accompanied us the whole time the Lord Jesus came and went among us, beginning from the baptism of John until the day on which he was taken up from us, become with us a witness to his resurrection." So they proposed two, Joseph called Barsabbas, who was also known as Justus, and Matthias. Then they prayed, "You, Lord, who know the hearts of all, show which one of these two you have chosen to take the place in this apostolic ministry from which Judas turned away to go to his own place."
Then they gave lots to them, and the lot fell upon Matthias, and he was counted with the eleven apostles. (Acts, i, 15-26)