By the time the first stories about Jesus and his followers were written, decades had passed since the events of his life, decades in which stories had many years to make the rounds, being told and re-told, changed, mis-remembered, amplified, enhanced and forgotten. Those who later wanted to record Jesus' life had nothing objective to work from, only stories passed around for years, for generations. As different people began to write versions of what they had heard, or versions of what they heard from others, differences and similarities began to emerge. They borrowed from one another. They added and excluded, sometimes changing stories to fit their perceptions and needs. The standard chronology and relationships among all of the Gospels are these:
Mark was composed before Luke or Matthew. We know this because there are stories in Luke and Matthew that are almost direct copies of stories that appear in Mark. Sometimes these stories are not verbatum repeats but have been changed to a greater or lesser extent from those same stories in Matthew, but they are obviously the same stories told in different ways.
All of the stories in Mark are not repeated in Luke and Matthew. In fact, there is much in Mark that does not appear in the other two Gospels. And, there are stories in Luke and Matthew that are the same or very similar to each other that do not appear in Mark. This means Luke and Matthew both had access to another source that Mark did not have. German scholars over a hundred years ago were the ones to have first published this finding. They this separate source used by Luke and Matthew's "Q", from the German quelle for source. Hence, what scholars now refer to as The Q Gospel. The generally accepted theory is that most of Q's content appears in Matthew and Luke, but the majority of what was in Q is in Luke.
To make things more interesting we know that each of the three synoptic (in synch with one another) gospels, Mark, Luke and Matthew, also drew upon sources distinct from each other. These other sources were in some instances gnostic gospels, those accounts not accepted by the Church as real or official, but heretical, or "wrong thinking" as opposed to orthodox, or "right thinking". Most of these heretical sources have names like The Sayings Gospel, The Gospel of Thomas, The Cross Gospel, The Gospel of Judas, The Gospel of Mary, to name the main ones we know about. Probably the oldest of these for which we have copies is The Gospel of Thomas that first came to light with the discovery of a library of ancient Coptic Christian scrolls at Nag Hammadi in Egypt in 1945. These were predominantly gnostic texts but also included a partial translation of Plato's Republic.
The Gospel of Thomas is radically different from any of our other gospels, canonical or gnostic. First, there is no birth story of Jesus and no chronology of events, only a listing of the saying of Jesus. One hundred fourteen of them. Second, there are no references to early Jewish prophesies, even though these are constantly interleaved with events in the canonical Gospels. Third, many of the Thomas sayings appear, often literally, in some of the canonical Gospels.
Thomas begins simply: "These are the secret sayings that the living Jesus spoke and Didymos Judas Thomas recorded." Didymos is Greek for twin. In some traditions, especially in the Eastern Church, Thomas is often perceived as Jesus' twin, whether biological or spiritual, I am not clear. If you wish to drill deeper into this bedrock, start with the Gnostic Library online
That leaves us the Gospel of John, an accounting that is so different from the other three official Gospels that one wonders why it was included at all. In John, Jesus is nothing at all like he is portrayed in the other three. He is likely to break into pages long speeches, convoluted tales and recitations of who he is and why he is here. He does this only in John. Scholars attribute this Gospel to a number of different authors, believing that it went through two or three iterations before settling into its current version, and derived from a community of later Christians who were drawing ever more apart from their Jewish origins. In fact, John is the most anti-Jewish of all the gospels, referring to Jesus' enemies simply as "the Jews."
In summary then, the official, sanctioned Gospels are the four books entitled Mark, Matthew, Luke and John. However, without other ancient and now almost forgotten sources, stories, gospels and traditions they would not exist. Behind the Gospels lurks a more rich, varied, complex and confused amalgam of stories from which the official line of the Church was forged. This is not an evil thing, or something to be kept hidden in the attic like your crazy great aunt Charlotte. Uncovering the truth behind history should be honorable and respected work, not something to be demeaned, blamed or swept under the rug.
If people think Jesus of Nazareth was an important person in the world, that he had a good word to say about life and the honorable and ethical way to live it, then it should be important to diligently, intelligently and objectively work to discover what those words, and his deeds, actually were. In any rational measure, the official canonical Gospels fail in that most important task.