For the edification of all:
That is from Roman Catholic Canon; not "New Advent", which is an unofficial Roman Catholic Website.
Critical analysis makes it difficult to accept the idea that the gospel as it now stands was written by one person. John 21 seems to have been added after the gospel was completed; it exhibits a Greek style somewhat different from that of the rest of the work. The prologue (John 1:1-18) apparently contains an independent hymn, subsequently adapted to serve as a preface to the gospel. Within the gospel itself there are also some inconsistencies, e.g., there are two endings of Jesus' discourse in the upper room (John 14:31; 18:1). To solve these problems, scholars have proposed various rearrangements that would produce a smoother order. However, most have come to the conclusion that the inconsistencies were probably produced by subsequent editing in which homogeneous materials were added to a shorter original.
Other difficulties for any theory of eyewitness authorship of the gospel in its present form are presented by its highly developed theology and by certain elements of its literary style. For instance, some of the wondrous deeds of Jesus have been worked into highly effective dramatic scenes (John 9); there has been a careful attempt to have these followed by discourses that explain them (John 5; 6); and the sayings of Jesus have been woven into long discourses of a quasi-poetic form resembling the speeches of personified Wisdom in the Old Testament.
The gospel contains many details about Jesus not found in the synoptic gospels, e.g., that Jesus engaged in a baptizing ministry (John 3:22) before he changed to one of preaching and signs; that Jesus' public ministry lasted for several years (see the note on John 2:13); that he traveled to Jerusalem for various festivals and met serious opposition long before his death (John 2:14-25; 5; 7-8); and that he was put to death on the day before Passover (John l8:28). These events are not always in chronological order because of the development and editing that took place. However, the accuracy of much of the detail of the fourth gospel constitutes a strong argument that the Johannine tradition rests upon the testimony of an eyewitness. Although tradition identified this person as John, the son of Zebedee, most modern scholars find that the evidence does not support this.
The fourth gospel is not simply history; the narrative has been organized and adapted to serve the evangelist's theological purposes as well. Among them are the opposition to the synagogue of the day and to John the Baptist's followers, who tried to exalt their master at Jesus' expense, the desire to show that Jesus was the Messiah, and the desire to convince Christians that their religious belief and practice must be rooted in Jesus. Such theological purposes have impelled the evangelist to emphasize motifs that were not so clear in the synoptic account of Jesus' ministry, e.g., the explicit emphasis on his divinity.
The polemic between synagogue and church produced bitter and harsh invective, especially regarding the hostility toward Jesus of the authorities--Pharisees and Sadducees--who are combined and referred to frequently as "the Jews" (see the note on John 1:19). These opponents are even described in John 8:44 as springing from their father the devil, whose conduct they imitate in opposing God by rejecting Jesus, whom God has sent. On the other hand, the author of this gospel seems to take pains to show that women are not inferior to men in the Christian community: the woman at the well in Samaria (John 4) is presented as a prototype of a missionary (John 4:4-42), and the first witness of the resurrection is a woman (John 20:11-18).
The final editing of the gospel and arrangement in its present form probably dates from between A.D. 90 and 100. Traditionally, Ephesus has been favored as the place of composition, though many support a location in Syria, perhaps the city of Antioch, while some have suggested other places, including Alexandria.
The principal divisions of the Gospel according to John are the following:
http://www.usccb.org/nab/bible/john/intro.htm
That is from Roman Catholic Canon; not "New Advent", which is an unofficial Roman Catholic Website.