Well both of those are honest questions, and I'll answer them with my own opinion of course. I don't deny others may have a completely different view on them but hey, this isn't their blog haha.
Okay, so lets tackle the first question...
"What is Christian Art?"
It would be very easy to say that Christian art is just art that projects the overall theme of Christianity (i.e. God, Jesus, the Cross, Angels, etc...) and you wouldn't be wrong at all because that is absolutely true. However, since Art itself is interpretive you could go deeper with it than that.
"We shall mount up with wings as eagles, we shall run and not be weary, we shall walk and not faint..."
When I read that I almost always picture an eagle soaring in the sky, and if I wanted I could pick up a brush and paint just that... an eagle soaring over a mountain top with the lush green forest underneath. That would be my interpretation of what I felt from that scripture.
Now I could add the scripture to the picture somehow (either directly to the picture or a separate small piece of maybe card stock paper overlaying it) and it would be very nice.
In my own opinion doing that OR leaving the picture just as it is becomes Christian art because the person who drew it was focusing not only on the picture but on where the inspiration was coming from.
Now there are a few things Christian art is NOT and never will be and those things fall more into the second question...
"How Is Christian Art Different Than Secular Art?"
This one can be answered very easily.
Christian art will never glorify sin. It's that simple.
You cannot glorify sin in art and then claim it to be Christian but that point should really already be understood. The bigger thing that separates Christian art and secular art is just the artist and the audience they market to.
I'm sure some think that isn't true and I'm sure an example can be made of how I'm wrong. However, I'll give my own example of how it can be right.
I could paint a net being pulled over a row boat, filled with fish to the max (not scripture reference included with the painting) but Christians who looked at it may be reminded of how Jesus said "I will make you fishers of men."
While a secular artist could paint the very same picture but his customers be reminded of a day they went fishing with their grandfather.
At the end of the day there is a big difference between Christian art and secular but that's only when you fully attempt to make it obvious. If you make the attempt, it becomes a given. No one will argue that Jesus on the cross is Christian art.
You can , however, be a Christian who is an artist and therefore you art becomes Christian art.