When asked what classical music is, many people define it as the old stuff their parents like. This is the wrong answer although it may be good for a chuckle or two. Classical music is a strictly defined set of musical compositions. They were European in nature and most were composed during an 80 year period beginning in 1750. At that time, sonata, symphony and concerto music was defined within the music community as true classical music.
The reason there had to be a definition was due to changes in popular music. Until that time, there was no difference between what everyday people heard and what was performed in theaters, music halls, churches or at special events. Musicians began writing music that was popular but not part of any larger work. Single pieces were written simply to be enjoyed by the general public. Leaders in the music world made the decision to define classical as opposed to popular music forms.