The Gospel of Luke was written by Luke, the beloved physician (Colossians 4:14). He was not one of the original 12 disciples of Jesus, but nevertheless came upon the Christian scene quite early. He was also apparently not a Jew by ethnicity, but rather a gentile. In fact he is the only non-Jew to write a book of the New Testament. Furthermore, Luke is the only gospel writer to produce a sequence to his gospel, namely the Acts of the Apostles.
Luke wrote the gospel bearing his name taking a scientific approach. This was quite natural for him as a trained physician. Not being an eyewitness to the things whereof he wrote, he interviewed reliable people. He "investigated everything carefully" and produced "an orderly account" of the life and teachings of Jesus Christ. It was different with the book of Acts. Luke became a co-worker with the Apostle Paul during his second and third missionary journeys and thus an eyewitness to the expansion of the early church as evidenced by the "we sections" (beginning with Acts 16:10).
The book of Luke itself is divided in the following way by Leon Morris in his Tyndale NT Commentary:
Luke, along with Matthew and Mark are called synoptic gospels, meaning "same gospels." That means, many of the verses are exactly the same in all three books. This suggests a common source ("Q" for the German word Quella meaning "source"). Of course, there are many differences as well in what is or is not included. A clear feature of Luke is his emphasis on the Holy Spirit. One senses as one reads this gospel that there is a deep spirituality.