The Epistle reading is always linked to a reading from the Gospel, though some services, such as Matins, will have a Gospel lesson, but no Epistle (Vespers on a number of feast days (typically for Apostles) will also have three epistle readings but no Gospel).
Epistles: The letters in the New Testament from Apostles to Christians are usually referred to as epistles.
Gospels: Four Books written by Evangelists (Matthew, Mark, Luke and John) about the life of Jesus