The word apostle comes from the Greek αποστολος, literally meaning "one who is sent out." Originally a military term referring to a sortie sent out against the enemy, apostle has in the Christian context come to refer to a missionary spreading the gospel of Jesus Christ.
Our Lord originally appointed twelve apostles as his initial Church community, commissioned with the task of sharing the faith. As told in the Gospels, Judas Iscariot was originally one of them, but with his apostasy and death, he was replaced with Matthias. Their collective feast day is June 30.
After Judas' replacement, the Twelve consisted of the following:
The Seventy Apostles are those whom the Lord chose (described in Luke 10:1-16) in addition to the Twelve and sent forth to assist in the work of preaching. Over time, the Twelve added others to their number, who were sent out with the original Seventy to preach the gospel. Although this number eventually exceeded seventy, they were all nevertheless referred to as "of the Seventy" out of reverence to the number which the Lord originally chose. Their collective feast day is January 4.
It is difficult to determine a comprehensive and accurate list of the Seventy, but here are some of their names:
Acaichus, Agabus, Alphaeus, Amplias, Ananias (who baptized Paul), Andronicus, Apelles, Apollo, Apphia, Aquila, Archippus, Aristarchus, Aristobulus, Artemas, Asyncritus, Barnabas (leader of the Seventy and companion of Paul), Caesar, Carpus, Cephas, Clement, Cleopas (who accompanied the Lord to Emmaus), Crescens, Crispus, Epenetus, Erastus, Evodus, Fortunatus, Gaius, Hermas, James (the Brother of God, also called "the Less"), Jason, Junia, Linus, Lucius, Luke the Evangelist (companion of Paul and author of the Gospel that bears his name), Mark the Evangelist (companion of Paul and author of the Gospel that bears his name), Nicanor (one of the original seven deacons), Olympas, Onesimus, Onesiphorus, Parmenas (one of the original seven deacons), Patrobas, Philemon, Philip (one of the original seven deacons), Phlegon, Priscilla, Prochorus (one of the original seven deacons), Pudens, Quadratus, Quartus, Rodion, Rufus, Silas (companion of Paul), Silvan, Sosipater, Sosthenes, Stachys, Stephen the Protomartyr (one of the original seven deacons), Terpnus, Tertius, Thaddeus (sometimes confused with Jude of the Twelve), Timon (one of the original seven deacons), Timothy (companion of Paul), Titus, Trophimus, Urban, Zacchaeus, and Zenas.
The Apostle Paul is also referred to as an apostle, though he is not one of the Twelve or of the Seventy. Other missionary saints are also referred to as apostles, though usually not as an initial title, but rather as an epithet, e.g., "Apostle to the North." Further, other saints have the epithet "equal to the apostles," usually because of their significant work in building up the Church.