The Gospels were all written by different men. In the case of the Gospel of Mark, it was written most likely by Mark, a companion of the Apostle Peter. Most historians agree that Mark is the oldest of the four Gospels, and therefore Mark provides some of the building blocks for the other 'synoptic' gospels (Luke and Matthew) which share some of the same content. As a companion of the Apostle Peter, Mark would have heard the Gospel first hand from Peter, therefore some people say that the Gospel of Mark is the Gospel that would have been preached by Peter.
The Gospel of Matthew is thought to have been written by the Apostle Matthew, the only apparently educated member of the twelve Apostles of Christ. The Gospel of Matthew is written with a particular emphasis on reaching the Jewish people and showing that Jesus is the Prophesied Messiah of the Old Testament. Importantly the Gospel of Matthew starts with Jesus' genealogy, which connects Jesus with his Jewish heritage.
Luke was a companion of the Apostle Paul, and was most likely a Gentile. He was trained as a Roman physician and educated in Greek rhetoric. In the Greek, Luke's Gospel uses the most compelling language for a first century Gentile living in the Roman Empire. When reading Luke, it is important to understand that Luke was written as the first half of one book, Luke-Acts, and they should be read and understood in tandem. Luke actually begins the Gospel by addressing his audience, Theophilus, most likely an educated, affluent Gentile background believer. In the case of Luke's Gospel, Luke emphasizes the historicity of Jesus' birth, death and resurrection by linking events in Jesus' life to contemporary secular events and people that a Gentile would have been familiar with. For example, in Luke chapter 3 when John the Baptist is introduced Luke makes sure to mention the secular leaders that were in power at the time to give a point of reference for the reader, "In the fifteenth year of the reign of Tiberius Caesar when Pontius Pilate was governor of Judea, Herod tetrarch of Galilee, his brother Philip tetrarch of Iturea and Traconitis, and Lysanias tetrarch of Abilene"
The Gospel of John is known to have been the latest Gospel to have been composed, it was most likely composed near the end of the life of the Apostle John while he was in exile on the Isle of Patmos. John had a very special relationship with Jesus, he was known as the Apostle whom Jesus loved. He was one of the three apostles who saw Jesus when he was transfigured, he was one of the three apostles who went with Jesus to pray at Gethsemane. His experience of Jesus was different than most of the other Apostles and Disciples. It is also important to understand that John was a young man when he was an Apostle, and had long outlived all of the other Apostles. He was an old man when he composed this Gospel, and most likely had read and had access to the other Gospel accounts written by Mark, Luke and Matthew. When John composed this Gospel, he wanted to fill in some gaps that were not covered in the other Gospels... he didn't set out to write a different account, just a different perspective. He was writing about Jesus not as a second hand account as in the case of Mark and Luke, but as a personal, first hand account of Jesus. Near the end of the Gospel of John, John even addresses the possible future question that will be raised about why his Gospel appears to be different than the others by saying, "And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen." (John 21:25) He is basically saying, "I could have composed countless Gospels about Jesus and wouldn't ever have to repeat myself, there is just so much to say!"
Each of the Gospels was about the same man, Jesus, and each of them see Jesus through their own eyes, and hear him with their own ears. Each of them perceives him slightly differently. And in the case of each of the Gospels, they are written with different audiences and issues in mind. Luke for Theophilus, a new believer. Mark for the churches that Peter was overseeing. Matthew for Jews. John, as a more 'spiritual' Gospel, emphasizing Jesus' divinity. But essentially, they were all penned under the authority of the Holy Spirit. Each of them speaks differently to each one of us, to the needs of our lives, but we must understand that Jesus is not four separate people, he is the Son of God, and each of these Gospels is a different point-of-view account of his life, ministry and saving work on Calvary.
Long answer to a short question.