A Gospel is a specialized biography written to proclaim the "good news" (Gr.: εὐαγγέλιον; anglicized: evangel). It is the message proclaimed by a herald, especially to tell of the coming of the king.
"The Gospels" is the section of the Bible containing the books of Matthew, Mark, Luke, and John that give accounts of the life and teachings of Jesus Christ. The Gospels make up the first section of the New Testament.
Though none of the Gospels are "signed" as are the epistles, the Christian Church agreed early on to the authorship of each of them. Knowing the author was one of the things that convinced the leadership that a book should be included in the books of the Bible.
The traditional order of the gospels was not established in the earliest writings, but the order in modern Bibles has often been accepted to be chronological. Modern scholarship points to Mark as the first one written due to its being 'contained' in the other two "synoptic" (from similar view) gospels. Nevertheless, no modern translations have adjusted the order.
Matthew was considered the author of the 'first' gospel from as early as the early second century. One clue to the gospel attributed to him as might be in the account of Matthew's being called as a disciple. Afterwords, the meal thrown in Jesus's honor is said to be 'in the house' rather than 'in his house' (comparing Matthew with Mark). Since Matthew was a disciple, his account would be the earliest eye witness of Jesus to be recorded.
Mark was not a called disciple of Jesus, and is not named specifically in the account with his name. However, his contact with Peter's ministry in Acts may indicate Peter was his 'source' for information. In addition, the style of this gospel is action-packed, apparently written for a Roman audience by a Roman ("Marcus"). As with Matthew, there may be a 'signature' verse. As Jesus was being taken into custody, an attempt was made to grab a bystander, a 'certain young man,' who ran away naked (having lost his bedclothes).
Luke is identified as a physician who traveled with Paul. As a result, the authorship of Luke and Acts (both written by the same person to Theophilus) can be determined by searching Acts for the so-called "we" passages. When the writer switches to the first person, the text will have indicated the addition of someone to the traveling group. This turns out to be Luke. Like Mark, he is a Roman (Luke = "Lucus"). Unlike Mark, Luke probably had not personally seen Jesus. His account had been the work of a master historian for the benefit of the Roman official Theophilus.
John called himself the disciple "whom Jesus loved" even when he was grouped with his brother James. A comparison to the book of Revelation reveals that the self-named disciple "John" that wrote from Patmos was the same person who wrote the epistles and gospel with that name. As one so close to Jesus, John's account of his time with the Master is much different than Matthew's.
All four Gospel writers testified to the works and teachings of Jesus.