Okay, actually, this post is about source criticism. What's the difference? I'm so glad you asked! When it comes to Biblical scholarship, there are a lot of critical fields, each of which focuses on a different aspect of the Scriptures. Textual criticism investigates the physical text, the words on the paper (or papyrus, or vellum), and tries to figure out which manuscript or reading is the most accurate. Literary criticism examines the Scriptures from the point of view of literature; for example, considering the poetic elements used in a passage. There are many more. Today, I thought I'd talk about source criticism, which examines the possible sources of a text. Who wrote a particular book of the Bible? Where did the author live? When was he or she writing?
One example of source criticism would be, who wrote the Pentateuch, the first five books of the Bible? They are traditionally attributed to Moses, but anyone who reads the end of Deuteronomy quickly realizes that Moses could not have written an account of his own death. Through this and many other more complex considerations, scholars try to determine the authorship of the Scriptures.
In the case of the Gospels, one of the major questions posed by source criticism is, "Which Gospel was written first?" Traditionally, Matthew was thought to be first, which is why it was placed first in the New Testament. Scholars today generally agree that Mark is the oldest Gospel, followed by Luke and Matthew (though there is disagreement about which of these two was written first), with John being the latest Gospel. Matthew, Mark, and Luke are referred to as the "synoptic Gospels", which means that—as can be seen easily from reading them—they are roughly parallel. John is the oddball; it includes stories none of the other Gospels include, and leaves other, sometimes very significant, stories out. One of the most striking examples of this is the Last Supper; John has no account of this meal with Jesus and his disciples.
The question came to be raised, "Why are the synoptic Gospels so similar, with John being so different from all three?" It would seem reasonable to assume that the Synoptic Gospels were drawing on similar material, such as stories passed down by word of mouth, or even that the authors were aware of one another's work in some way. The most popular (though certainly not the only) theory regarding the Synoptic Gospels is called the "Two-Source Hypothesis", along with its closely related cousin, the "Four-Source Hypothesis". These theories assume that Mark was the earliest Gospel, and that Matthew and Luke both knew Mark's Gospel when they wrote their own accounts. This would explain why most of the stories in Mark are repeated in Matthew and Luke. However, there is also material found in both Matthew and Luke which is not in Mark, suggesting another common source used by Matthew and Luke, but not Mark. This source, which has never actually been found, is theoretically known as Q (from German Quelle, meaning "source"). Mark and Q, thus, are the two sources of the Two-Source Hypothesis. Since Matthew and Luke also have material unique to their Gospel accounts, some scholars imagine that they had other sources, possibly oral, possibly written, which were not shared by the other Gospels writers. These sources are referred to as M, for Matthew's source, and L, for Luke's. (source criticism does not require creativity in coming up with these names). Adding M and L to Mark and Q gives you the Four-Source Hypothesis.
Again, this is a very simplified explanation, but I think it gives a pretty good overview of source criticism and the understanding of how the Gospel sources relate to one another. I promise, I won't keep doing Biblical criticism posts all semester.