Not every story of Jesus is included in every gospel; several well-known ones (like the Prodigal Son) were absent from Mark and Matthew, so they must be in Luke and/or John. Luke is the last of the 3 synoptic gospels (those that are mainly biographical and share source material).
The gospel of Luke and the Acts of the Apostles (which follows John) were written by the same unnamed author. Together they account for over one quarter of the New Testament, providing the framework for both the Church’s liturgical calendar and the historical outline of the story of Jesus. Luke was written between 80-100 CE, and revised further after that, but it’s not entirely clear whether it was written after Matthew or around the same time. The Holy Spirit plays an important role in Luke, and there is emphasis on loyalty to God rather than to the rulers of this world, who are alleged to take their power from Satan.
The last gospel, John, dates to around 90-110 CE, but was likely written in stages during that time. Early church literature identifies the author as John, the disciple of Jesus, who also wrote 3 epistles and the book of Revelation. However, modern scholars doubt that Jesus’ apostle John wrote any of these books, and authorship remains unknown. I can’t say I’m surprised.
While the first 3 gospels are quite similar, 90% of John is unique. Here are some of the differences to watch for as we read along: John is more theological then biographical, with Jesus talking openly about his divine role. John talks a lot about the relationship of the Son to the Father. It also focuses on the relation of the Redeemer to believers, the Holy Spirit as the Comforter, and the prominence of love in Christian character. Jesus’ teachings in John are largely irreconcilable with those found in the other gospels, and many scholars consider the Synoptic Gospels to be more accurate representations of the teachings of Jesus (if there even was a historical Jesus). And sadly, John is the primary source of the image of the Jews persecuting Jesus.