From Greek epistolē ("letter, message sent"). The 21 New Testament letters — 13 from Paul (or 14 if Hebrews is included), 1 each from James and Jude, 2 from Peter, 3 from John — form the largest single genre in the NT. Epistles were written to specific audiences (particular churches, groups of churches, or individuals) to address particular situations, doctrinal issues, ethical problems, or pastoral needs. They follow the general conventions of first-century Greek letters (sender, recipient, greeting, thanksgiving, body, final instructions, closing benedictions) but transform those conventions with theological depth and apostolic authority.
Reading the epistles well requires noticing several features. (1) Occasional documents. Every epistle was triggered by a specific situation on the ground: the Galatians were being told to be circumcised; the Corinthians were divided and immoral; the Colossians were facing a proto-Gnostic heresy; Timothy was leading a church in Ephesus facing false teaching; Philemon had to decide what to do with a runaway slave. Reading an epistle without its occasion is like overhearing half of a phone conversation — much can be missed. (2) Indicative before imperative. Most epistles (especially Paul's) present the theological foundation first (the indicative — what Christ has done, what you are in Christ) and then draw out the ethical implications (the imperative — what you should therefore do). Ephesians 1-3 before Ephesians 4-6; Romans 1-11 before Romans 12-16. Flipping this structure turns the gospel into moralism. (3) Fellowship of apostolic authority. The epistles are not merely wise counsel from senior Christians — they are the authoritative written voice of the apostles, carrying the authority of Christ Himself (2 Peter 3:15-16 equates Paul's letters with "the other Scriptures"). (4) Preserved for the Church universal. Though occasional, the epistles speak beyond their original audiences to every church in every age dealing with parallel issues. The Holy Spirit intended these letters for us as well.