Answer
Notionally, epistles are simply letters. In the New Testament, there are at least three classes of epistle:
- Open letters from an apostle such as as Paul, usually to a church or group of churches. Romans is an excellent example of this type of epistle.
- Pseudonymous epistles, written by anonymous authors as if by an apostle of earlier times. The Pauline epistles to Timothy are examples of this genre as is, for example, Jude.
- Encyclicals, which are not really letters, but pronouncements intended to copied and circulated widely. Ephesians, which is also a pseudonymous epistle, is considered by some scholars to be one such example, with the opening address to the Ephesians probably a late addition. Hebrews may also fit this category, although it may actually have been a sermon rather than an encyclical.