The first three gospels are sometimes known as the "synoptic" gospels. What does this term mean, and how does it differentiate them from the gospel of John?
Syn- means same (as in synchronize, same time), and optic of course refers to sight. The three first Gospels are syn-optic because they see alike; they basically tell the same story as each other, covering many of the same events. John, on the other hand, focuses on a lot of different material than the other authors, and has a very high percentage of unique content.
A brief look at any harmony of the Gospels will immediately point out an obvious fact - namely, Matthew, Mark, and Luke go over a lot of the same ground, but John is very different.
For the uninitiated, a harmony of the Gospels is a work that attempts to show the life of Christ in chronological order, pointing of the reference texts.
The number of parables and stories that occur in Matthew, Mark, and Luke, which do not occur in John, is just too great.
Note, for example, just the miracles of Christ.
Miracle Matthew Mark Luke John
The nobleman's son at Capernaum healed 4:46-54
The demoniac in the synagogue healed 1:21-28 4:31-37
Simon's wife's mother healed 8:14-17 1:29-34 4:38-41
Circuit round Galilee 4:23-25 1:35-39 4:42-44
Healing a leper 8:1-4 1:40-45 5:12-16
Christ stills the storm 8:18-27 4:35-41 8:22-25
Demoniacs in Gadarenes 8:28-34 5:1-20 8:26-39
Jairus' daughter. Woman healed 9:18-26 5:21-43 8:40-56
Blind men and demoniac 9:27-34
Healing the paralytic 9:1-8 2:1-12 5:17-26
Matthew the publican 9:9-13 2:13-17 5:27-32
"Thy disciples fast not" 9:14-17 2:18-22 5:33-39
Because these three seem to have the 'same view' on a lot of matters, they have been called the "synoptic" Gospels, from the Greek syn (same) and optic (relating to sight or view).
Famed exegete Daniel Wallace goes into detail on the so-called Synoptic Problem, asking how it is that three Gospel writers could include so much that is so similar, in even the same order. While he does not deny inspiration by the Holy Spirit, he shows how the complete agreement would not really fit with three men independently relating the same facts, without some form of collaboration or borrowing. A close reading of parenthetical remarks or the details which are included in some accounts but not others would seem to indicate that the authors were aware of the other works, and expected their audience to do the same.
Given this similarity, the fun question then becomes, who borrowed from whom?
Traditionally, Matthew was viewed as the first Gospel, hence its location first in the New Testament canon. That said, beginning with Schleiermacher in the early 1800s, the idea that Mark was actually the first began to take hold.
The rules of hermeneutics that are used to reconstruct "the original wording" (remember these were hand-copied manuscripts and minor variations happened) usually suggest that when two possible variations are presented, "the harder meaning is to be preferred." In other words, if a copyist is reading sourcing material and either re-copying it or re-using it, it is far more likely that a copyist would harmonize what is said with an overall theology, rather than introduce something that makes the existing theology less pure and pristine. Since Mark's Gospel typically includes some of the most human aspects of Jesus (the Messianic secret, the abrupt ending, etc...), the principle is taken to a macro level, thus giving Mark "temporal first place."
The synoptic "problem" does not seek to deny "Inspiration," but does seek to create an historical chain of events that tells us how our Gospels came to be.
To contribute to the exceptional answers already here with a bit of additional information, the source material referred to as Q (or Q source, or Q Gospel, name from the German Quelle) is held in common between both Matthew and Luke, in addition to the common material from Mark, which is presumed to be written first. That said, there is debate on the nature and even the existence of the Q source, being theology. A good article about Q source can be found in the Anchor Bible Dictionary, though this may be difficult to access for those without academic theological libraries nearby.
The reason for the term synoptics is that they can be viewed together to gain parallel understanding of the three Gospels. John, however, is not considered synoptic because it has many different events (whereas Matthew, Mark, and Luke have the same basic events with some additional or missing events based upon source materials) and different structure and order.
An important caution in regards to the synoptics is not to proof text synoptic parallels. It is common for passages to be matched up in synoptic parallels based upon similar general meanings when translated, which may be accurate, or may not be, based upon the original context of the passage. The heretical Gospel of Thomas (deemed as a Gnostic text that promoted a secret knowledge of Christ) is found at many points to have parallels with the synoptics also, though itself nothing like the other Gospels in structure.
Matthew, Mark and Luke are very similar: they record many of the same miracle stories, parables, and sermons. John by contrast has fewer miracles (most of which are unique), no parables at all, and is the only Gospel to record Jesus's teaching on the nature of God at the Last Supper (ch. 13-17). Synoptic comes from the Greek for "see together" because they tell the Jesus story in the same way.
When the first three gospels are compared—Matthew, Mark, and Luke—it is obvious that the accounts are very similar to one another in content and expression. As a result, Matthew, Mark, and Luke are referred to as the “synoptic gospels.” The word “synoptic” basically means “to see together with a common view.”
Not that they should contradict each other but the striking similarities, not only of wording, as for example in the Healing of the Paralytic (Mk.2.1 12; Mt.9.1 8; Lk.5.17 26.), but also of the order of incidents, as for example the series of episodes recorded in Mark 10.13 34, and appearing in Matthew 19.13 xx.19 and Luke 18.15 34 presented a literary puzzle of sort. The question that arise is why these similarities?
Plausible explanation for similarities in order of events could be that writers presented them in the order of their occurrences. However for similarities of wording most of the literary studies only discuss this issue without taking into consideration the influence of Holy Spirit and point out to the literary dependence of these gospels on each other, which definitely the most logical conclusion one can expect with a sceptic mind.
One such research however bases this on oral traditions and concludes that the synoptic gospels are probably related to one another in a mostly literary, partly oral, fashion. It further goes on to say that it is highly likely that they were written from mostly oral traditions. However if the gospel authors had composed entirely from oral materials, each successive writer still depended on the groundwork of his predecessor(s).
Another study which is staunchly advocating the inspirational base for these similarities has found that only sixteen percent of the words in those sections are identical in all three Gospels. The study then focused on the agreements of two Gospels against a third Gospel in all combinations: Matthew-Luke against Mark, Matthew-Mark against Luke, and Mark-Luke against Matthew. It showed from several perspectives the impossibility of any theory of literary interdependence created by these combinations of agreements and disagreements. From the standpoint of observational facts, it pointed to randomness as the only possible explanation for the phenomena of the Gospels. Yet randomness is not an accurate term to apply.
As already said, the discussions of the origins of the Synoptic Gospels say far too little about the role of the Holy Spirit in the composing of those Gospels. He was the major author in the divine-human process of producing the Synoptic Gospels. Some of Jesus’ last words to the Eleven were:
“When the Holy Spirit comes, whom I will send to you from the Father, the Spirit of truth who proceeds from the Father, that one will testify concerning Me; and you also will testify, because from the beginning you have been with Me” (John 15:26-27).
The study further says:
Those words specify the twofold nature of the inspiration that produced the accounts of Jesus’ life. “Randomness” is not a fit description of the combination of coincidences and disagreements in the Synoptics. The Holy Spirit had a controlling role in what the human authors wrote. He had reasons for the occasions when they agree and for the occasions when they disagree. In that sense, the combination of agreements and disagreements is not random, but God-ordained. In this life, we as humans will never comprehend the mind of God (cf. Isa 40:13; 1 Cor 2:16) and be able to detect His reasons for this mixture of agreements and differences in wording. To think that we can do so by treating Scripture as just another human production smacks of egotism on our parts. Readers must content themselves with simply accepting what He used the writers to compose while they worked without consulting the written works of each other.
The first three gospels (Matthew, Mark and Luke) are sometimes known as the "synoptic" gospels because by laying them in parallel and reading them synoptically ('with the same eye') in the original Greek language, it can readily be seen that there is a literary relationship among them.
Scholars have established that Matthew and Luke were substantially copied from Mark, with Matthew using some 90 per cent of the verses in Mark, often using the same words in the Greek language. Further sayings material not found in Mark but common to Matthew and Luke is attributed to the hypothetical 'Q' document, and once again, this could only come from an earlier, written Greek language document. Of course, the context of this is that the gospels were originally anonymous and, in spite of later attributions, we do not know who really wrote them.
There are several ways to establish independently that Mark was the original source gospel used for the other two gospels, rather than either Matthew or Luke. One, which John Dominic Crossan has studied, is a peculiarly distinctive Markan compositional device called an intercalation or sandwich. In The Birth of Christianity, page 106, he says that the device does not move from either Matthew or Luke into Mark but from Mark to the other two. Of Mark’s nine sandwiches, Matthew retains Mark’s pattern five times and Luke retains it four times, such that the only conclusion must be that in copying passages from Mark, the other two evangelists sometimes copied the intercalation pattern and sometimes not.
On page 565, Crossan identifies at least one instance in which John's Gospel also accepts an intercalation from Mark, but it is generally accepted that Mark was not the main source used by the author of John. The greater parallel in John is with Luke, but John contains many elaborations, changes of chronology, and passages for which the original source is unknown.