Although we are not told in the gospel who the author is, we can be pretty confident it was the apostle John, one of the sons of Zebedee the fisherman.
John’s gospel has a different feel to the gospels of Matthew, Mark and Luke (which are often referred to as the synoptic gospels, because they have the “same view”). John gives us more details of Jesus teaching concerning who he is. For instance, it’s in John we find Jesus’ “I AM” sayings in which Jesus declares “ I am… the light of the world… the good shepherd… the way, the truth and the life… “. In John we find an emphasis on certain themes such as light and darkness, Jesus as the “Word” and revealer of God, Jesus as the Son of God, who enjoys a oneness with the Father, and so on. It seems that John’s gospel was written independently of the others – whereas much of what Matthew, Mark and Luke write appears to come from the same source (perhaps being based on Mark’s gospel). John is therefore very helpful in giving us a different perspective on Jesus, who he is and what he did. In fact sometimes it is John who explains some things left unsaid in the other gospels.
Why did John write his gospel. As an eyewitness to Jesus life and teaching no doubt he wanted to preserve an accurate account for us. In particular, John wants to help us understand who Jesus is, that he is the Messiah and the Son of God – and what that means. However, John doesn’t simply want us to grow in intellectual knowledge of Jesus, he wants us to come to a saving faith in Jesus and to follow him as the Son of God (John 20:31). For John especially, belief in Jesus is given reality in changed lives.