Theory of Music is all about trying to understand how existing music works and how new music could or should be organized. Someone who makes a special study of music theory is a music theorist.

People who make their own music are composers. People who play or sing music are “performers”. It is important for both composers and performers to understand what makes the music sound the way it does. In the times of the Ancient Greeks the famous philosopher Pythagoras tried to explain how instruments are tuned. He understood the science of the good vibrations that the instruments make and explained how and why the octave is divided into twelve parts(in some cultures).