Pretty simple definition: Fantasy written by and for Christians. It is a story that reflects some part of the Christian world-view. Some Christian Fantasy is overtly Christian with strong imagery and the deliberate naming of Christian figures (God, Jesus, Moses, Mary, etc.). Other stories are more subtle with authors weaving only particular aspects of Christianity into the narrative.
Generally low. Magic is not a part of Christian world-views and so doesn't always find its place in its stories. Even in stories where magic is present, it is not magic in the traditional Fantasy-sense, and it is often tied into the glory of God.
High. Christian Fantasy stories grapple with some big ideas: creation, good vs. evil, morality, sin, faith, and finding meaning. There is always a theological idea to be explored or a lesson to be learned.
High. Characters in Christian Fantasy often undergo some kind of transformation or reach an understanding about the universe. As such, characters have well developed arcs in which readers get to see them change and evolve.
Variable. Christian Fantasy sometimes runs into a problem with reusing the same plots over and over again—sort of inevitable when there is a single central text influencing all stories. Then again, other writers succeed at crafting engaging plots with unexpected twists. Generally, the plots of Christian Fantasy stories have a pretty standard flow—they do not try to reinvent storytelling. There is standard rising action and resolution in these stories. Readers are not likely to be left hanging (unless the story gets picked up in another book in the series).
Moderate-High. Battling evil in one incarnation or another is a common trope of Christian fantasy, and these battles can be quite violent. Although, some authors will choose to skip the gory details of violent situations.
By C. Dale Brittain. This series incorporates elements of Christianity, but doesn't overtly involve Jesus.
By C.S. Lewis. An incredibly popular series with bot secular and religious audiences. For a series with witches and talking animals there are some remarkably strong Christian undertones woven throughout the stories.
By Jeffrey Overstreet. This series is not overtly Christian, indeed there isn't even an overt God-figure. However, there some distinctly Christian ideas about sin and creation.
By Peter James Dudek. An epic story of good and evil, of light and shadow.
By Brae Wyckoff. The holy city disappeared hundreds of years ago. This is a story about a spiritual battle and an epic quest. It is an interesting partnership of fantasy and theology.
By Jerry B. Jenkins and Tim LaHaye. A popular series about the Christian end-times that has been made into film and even a video game.
By Ted Dekker. This spiritually inspired series focuses on a man, who when he goes to sleep wakes up in an alternate reality
By Ray Owen. Spiritual and supernatural undercurrents are not overpowering in this book about several misfit characters. A new take on the use of angels.
By Alister E. McGarth. These books draw specifically on Christian stories and the characters have journeys toward faith in an alternate world.
By Karen Hancock. An example of allegory, this is a story about a life changing journey and good vs. evil.