The word gospel comes from the Old English godspel, "good news." It has the same meaning as the Greek evangelion—literally a "good message" or "good tidings"—the word used for Jesus' teachings in Greek, the language in which the New Testament was originally written.
In the years immediately after Jesus' ascension, historians state, stories of his life circulated primarily by word of mouth; different communities of the evolving Christian movement had varying collections of narratives and sayings. Though Jesus' crucifixion and resurrection took place in A.D. 30, the first of the four canonical gospels (which most scholars believe to be Mark's) was not written unril about forty years later. (Early Christians considered the Jewish scriptures to be their holy books; and, since many expected the imminent return of Jesus to earth, they apparently felt no need for additional written teachings. Though the first epistles, or pastoral letters, of the Apostle Paul were written around the year 50 or 52, no systematic narrative of Jesus' life and teachings was written until Mark's appeared around the year 70.)
Historians generally agree that both Matthew and Luke drew from Mark; and both of them, in addition, had access to material from other sources. Scholars attribute further differences in the content and emphasis of the four gospels, which, were written at different times between A.D. 70 and 90, to the particular events and concerns faced by the independent Christian communities—some of Jewish ancestry, some of Gentile—scattered in different regions of the eastern Roman Empire as the movement gradually evolved from its Jewish roots to a fully separate religion. (The Gospel of Thomas, also cited herein, is thought to have been compiled during the same period as the four canonical gospels—within a few decades after Jesus' ascension.)
Regarding the authors of the four New Testament Gospels, Matthew and John, of course, are names of two of the original twelve apostles of Jesus. Mark, according to Papias, bishop of Hieropolis in Asia Minor (ca. 130), was the assistant and "interpreter" of Jesus' disciple Peter; and wrote down what he heard Peter relate about the sayings and deeds of Jesus. Luke is identified in early accounts as the traveling companion of Paul, who had personally known Peter, John, and Jesus' brother James. An overview of what historians know about the authorship of the Gospels may be found in Three Gospels (New York: Simon and Schuster, 1997), by Duke University professor Reynolds Price.