Located between Acts and Revelation are the 21 books known as the Epistles or Letters. The format of these books are still familiar to us today. They include a greeting that addresses the recipient, the main body of the message and a blessing of good will in the end. The end may even include a post script (P.S.) that addresses other individuals.
13 of the 21 were written by Paul and include both his 4 Prison Letters and 3 Pastoral Letters.
8 letters were written to a general audience which is often referred to as the Catholic Epistles which simply means "Universal Letters", and has nothing to do with the Catholic religion but by name alone.
"letter" or "message"