As to what classical music is, Eric Friesen of Studio Sparks on Radio 2 has defined classical music—and I like this definition, but there may be many other definitions—as that which remains when all others have passed away. I think classical music is the timeless things that stay with us throughout history.
We don't have a measure of what that is, but we can tell a Matisse from a velvet Elvis, and we can know that one is a great work of art that will remain forever and one is...not necessarily. We can't say for sure that it's not.