There are four Gospels in the New Testament—books that tell the story of Jesus’ life—of which Luke is the third: Matthew, Mark, Luke, and John. (The same man, Luke, also wrote the book of Acts, and the two books are sometimes referred to together as Luke-Acts, although, unlike the books of 1 & 2 Chronicles or 1 & 2 Kings, the Gospel of Luke and the book of Acts were written as separate books, they weren’t just pulled apart later.)
Scholars believe Luke was probably a companion of Paul, and there are times in the book of Acts when Luke uses the word “we” in certain instances, indicating that he was present for those events. Luke wasn’t an eyewitness of the things written about in this book, but he did interview many eyewitnesses to ensure that he got his facts straight. In fact, he states that he wrote the book for a man named Theophilus, so that he could be certain that the things he was taught about Christianity were correct (see 1:1–4)—though I don’t know who Theophilus was, or why Luke felt he needed to offer the man this assurance.
In fact, not only was Luke not a firsthand eyewitness, he wasn’t even Jewish (as the majority of Jesus’ initial disciples would have been), he was a Gentile. This would have made Luke’s Gospel especially helpful for other 1st Century Gentile Christians; he wasn’t writing specifically to them, per se (though for all I know Theophilus was probably a Gentile), but it still would have been helpful that one of their best sources about Jesus’ life and ministry was written by a fellow Gentile.
Luke fills in more details about Jesus’ early life than the other Gospels do, so much of what we celebrate at Christmas comes from Luke’s Gospel. Luke also emphasizes Jesus’ social impact more than the other Gospels.