St. Paul, St. Peter, St. John, St. Jude all wrote epistles. St. James also wrote an epistle but it is uncertain if he was James the greater, James the lesser or another James.
The Apostle Paul wrote most of the letters in the New Testament. But also some were written by the apostle Peter and the apostle John
Often they are called "epistles".
http://www.interfaith.org/christianity/apocrypha/
The early leaders of the church decided the canon of the New Testament. These early leaders were Bishops in a number of cities in the Middle East.
Letters were written in the New Testament to address issues, provide guidance, and offer encouragement to the early Christian communities. They were also used to clarify theological beliefs, establish church practices, and strengthen the faith of believers. These letters were written by apostles and other early Christian leaders to help spread the teachings of Jesus and maintain unity among the diverse Christian communities.
The "Epistles" are letters from an Apostle to church members in other locations.
The New Testament is a history of selected events in the early church and is composed of 20 letters to churches and individuals and 1 apocalypse. The letters dealt mainly with the interpretation of God's act of salvation in Jesus Christ. Matters of discipline, proper Christian behavior, and church polity also are included. The apocalypse is a coded message of hope to the church of the first century that has been reinterpreted by each succeeding generation of Christians for their own situations.
Three names for early church leaders in history are Peter, Paul, and James. Peter was one of the twelve apostles and a prominent leader in the early Christian church. Paul was a missionary who wrote many of the letters in the New Testament and played a key role in spreading Christianity. James, the brother of Jesus, was a leader in the church in Jerusalem and played a significant role in the early Christian community.
The Apostle Paul wrote most of the New Testament as letters to different churches. For instance: Galatians was written to a church in Galatia; Phillippians to a church in Phillippi; Corinthians to a church in Corinth, etc.
They are called epistles
During Paul's time, there was no New Testament Bible. Paul was trained as a Jew so he would have studied the Law and the Prophets, or the Jewish scriptures. This is what Christians refer to as the Old Testament. Paul and the other apostles wrote Letters to churches. These letters were collected into the Bible based upon their acceptance by the church leaders. This is very general description but I hope it answers your question.
------------------------ The apocryphal books of the Bible were originally included in the Christian Old Testament when the Christian leaders did not really know which books the Jews regarded as sacred. Eventually it was discovered that these books were among those not included in the Jewish canon. The apocrypha were included in the original 1611 version of the King James Bible, but were removed from the 1666 version. Even the Catholic Church regards these books as 'Deuterocanonical', or second-canon. Although regarded as inspired, they are not inspired in the same way as the principal books of the Old Testament.
Paul did not preach or write any of the Gospels. He wrote several letters to the different Church leaders throughout the Roman Empire that later became scripture.
Jesus is to the leaders of the church, as the head of the church.