The book of Luke, named after its author, is one of the four Gospels, biographies that focus on the life, ministry, death, and resurrection of Jesus. There are many parallel passages in the gospels of Matthew, Mark, and Luke.
The book of Luke contains details and information not included by Matthew, Mark, and John (the other three biographies of Jesus) serving to nicely round out and fill in our understanding of Jesus.
Both Luke and Acts are written to Theophilus who may have commissioned the work.
There are many parallel passages in the gospels of Matthew, Mark, and Luke, but not too many with the Gospel of John.