We associate a number of writings in the New Testament with the name of John. There is of course the Gospel of John and the book of Revelation. Then there are the books we call 1, 2, and 3 John. Tradition associates all of these writings with a single person, the disciple John, the son of Zebedee.
Only one of these writings actually gives us the name John for its author, and that is the book of Revelation. The Gospel of John names an anonymous, "Beloved Disciple," as its chief source (e.g., John 21:24). The letters of 2 and 3 John only say that their author is "the elder." 1 John is more like a short sermon than a letter and has no names in it at all.
On the basis of style, the Gospel of John, the sermon of 1 John, and the letters of 2 and 3 John all have strong similarities. They also share some of the same themes. By contrast, Revelation is quite different, both in style and in the flavor of its content. Even in the earliest days of Christianity, these striking differences in tone and flavor were noticed. Dionysius of Alexandria, writing in the 200s, convincingly showed the likelihood that Revelation had a different author than the Gospel, letters, and the sermon we call 1 John. 
Dionysius even mentions that there were two tombs for John at Ephesus and suggests there might have been two Johns, one of whom wrote Revelation and the other of whom wrote the Gospel of John. This theory is strongly supported by the fact that another church father, Papias, also mentions two Johns, and he wrote as early as AD110! He even knew one of them. He calls one of these Johns a disciple, but the other he calls, "the elder." 
You can see why a few scholars in modern times have wondered if in fact the disciple John stands behind the book of Revelation and this other, "John the elder," stands behind the Gospel of John, the letters, and the sermon of 1 John.  Who would he be? Some have looked for clues especially from the Gospel of John. For example, the Gospel of John has a lot more to say about Jesus in the south, in Judea, than the other Gospels do. Could this John have been someone from Jerusalem?
The Beloved Disciple is known by the high priest (John 18:15). This is not likely something that would have been true of John the son of Zebedee, a fisherman from Galilee. But it might have been true of John the elder, perhaps someone from a priestly family.  The existence of two Johns would also explain how easily all these writings came to be connected to each other.
You can also see hints in the letters and Gospel of John of the situation around the area of Ephesus that was unfolding at the end of the first century of Christianity...
 Dionysius' contrast is found in the Ecclesiastical History of Eusebius, 7.24-25. His arguments are as good as any literary analysis by a modern scholar.
 It is probably worth pointing out that this is a relatively conservative hypothesis, since many scholars would disassociate the Gospel of John from any immediate follower of Jesus. By contrast, this is a reasonable explanation based on sound evidence that still takes seriously the earliest traditions of the second century. It may not be correct, but given how obvious the differences in style and approach are between John and Revelation, we have to consider how strongly some oppose this line of thinking as a certain kind of stubborn traditionalism. In the end, such vehement opposition reveals a lack of interest in the truth rather than, as is sometimes professed, a zeal for the truth.
 See especially Martin Hengel, The Johannine Question, trans. by John Bowden (London, SCM, 1990), and Richard Bauckham, The Testimony of the Beloved Disciple: Narrative, History, and Theology in the Fourth Gospel (Grand Rapids: Baker Academic, 2007), 50.
 Another writer named Polycrates, writing in the mid-100s, suggests that John was from a priestly family. It is tempting to think of John Mark, who was from Jerusalem. The difficulty here is that we already have a Gospel traditionally ascribed to Mark and the styles and approach of these two are almost diametrically opposite each other. Ben Witherington III suggests that the author of the Gospel of John might have been Lazarus, since John mentions how Jesus loved him. See John's Wisdom: A Commentary on the Fourth Gospel (Louisville: Westminster John Knox, 1995). This is ingenious but completely unattested by any early tradition.