They were apparently letters to churches and individuals written to handle specific problems at a church location or with a religious leader.
Unlike the Old Testament, which covers hundreds of years of history, the New Testament only covers several decades, and is a collection of the religious teachings and beliefs of Christianity.
The New Testament is not a single book written by one person, but, rather, a collection of twenty-seven books written in Greek by people from various places.
Historically, the word was first used by the church of those doctrines that were accepted as the rule of faith and practice.
The term came to be applied to the decisions of the Councils as rules by which to live.
The fact of the inspiration of the Bible as God’s special revelation to man naturally leads to the question (since many other religious books were written during both the Old and New Testament periods) what particular books are canonical, that is, what books are inspired and should be recognized as a part of God’s authoritative revelation? Are any books included that should not be in our Bible?