What kind of music is the classical music?

The word "classical" is used in two different ways in music.
The first refers to the style of most music written in the time approx 1750 to 1820 (of course, the change was gradual). Notable composers of this time are Mozart, Haydn, early Beethoven and Schubert.

The other sense refers to a much broader style of music.

about 90 years ago, jazz became a distinct style of music, separate from all the music that was around then (which is what we now call classical), and in jazz, all modern popular music has its roots.

So classical music is essentially all music that is not "popular" music. It tends to use acoustic or orchestral instruments (although in the last 50 or so years, this has begun to change).