John’s gospel is completely different from the Synoptic gospels.  It doesn’t even mention the best-known parts of the Synoptics, like Jesus’ birth, the temptation in the wilderness, Jesus’ Parables, the Sermon on the Mount, most of Jesus’ teaching about the Kingdom of God, or the Last Supper.

Matthew, Mark, and Luke were written within 20 years after Jesus’ death, and partly based on talking with people who had known Jesus.  John’s gospel was written as much as 60 years after Jesus’ death, so he had even more time to reflect on everything he had personally experienced, as an eyewitness and as part of Jesus’ inner circle.  During those years, John had identified the pictures and symbolism that related to Jesus’ life and mission, so signs and symbols make up the core of his gospel. 

John introduces his principal themes in the prologue, a passage that’s filled with symbolic meaning:  Jesus is God, and he came to earth in the flesh to bring light into our dark world, and to give everyone who believes in him the right to become children of God.

The true light that gives light to everyone was coming into the world.  He was in the world, and though the world was made through him, the world did not recognize him.  He came to that which was his own, but his own did not receive him.  Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God — children born not of natural descent, nor of human decision or a husband’s will, but born of God.  The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the one and only Son, who came from the Father, full of grace and truth.

John 1:9-14 NIV

The essential purpose of all four gospels is to encourage people to believe, and to inherit eternal life.  John makes that explicit:

Jesus performed many other signs in the presence of his disciples, which are not recorded in this book.  But these are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name.

John 20:30-31 NIV

This purpose is also found in what is probably the best-known verse in the Bible.  Jesus came to give eternal life to anyone who believes and accepts it.

For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.

John 3:16 NIV