What is classical music and how does it differ from other music?

"Classical music", strictly speaking, is music written in the years between 1750 and 1820. Progenitors of the classical style include W.A. Mozart, of course, Haydn, and Beethoven. It's a light and very logical style, full of consonant harmonies and easy to listen to. However, I think you are probably asking about that music that is not the so-called "popular" music, i.e. jazz, rock, hip-hop, etc. The ways in which that music differs from popular music is mostly in how it is handled and is a very, very long discussion. I highly suggest that you check out the Wikipedia article on classical music, and then narrow your question. http://en.wikipedia.org/wiki/Classical_music