What is music theory? 
Music theory is the study of music; and while the term is often applied to the Western musical tradition (more on that later), there are also music theories in other traditions around the world.
It is of interest to the reader to know that music theory emerged after the actual production of music. There were people making music long before we had ideas of what a major third is, and there were groups singing tribal songs long before we got the idea of a "downbeat". Although these ideas were often practiced before they were understood, to have a knowledge of what they are and how they work reduces their difficulty and increases their potential and flexibility.
What is music? 
Music can be many things, and the subject of what constitutes music has been long debated and argued, we will not be spending considerable time on its definition. Like all art, music is highly subjective and traditional. It is hard to make sense of music from an alien culture or tradition. This book only deals with a subset of music; a very large subset though. We look at the Western Tradition of music as it has evolved from the ancient European civilizations with influences from Africa, the Middle East and Asia. We will also look primarily at written music, rather than music which is only ever performed, but we realize that music notation can almost never capture everything that makes a piece. Sheet music is a limitation that we live with. You will notice the words "sometimes", "usually", "probably", and other similar terms used frequently as music is highly subjective and many "rules" are broken.
Music usually has four basic aspects: rhythm, melody, harmony, and timbre.
- Concerns note length, speed and stress — the "time" aspect.
- Concerns a series of notes — the "horizontal" aspect.
- Concerns notes played at once — the "vertical" aspect.
- Concerns the physical sound of notes — the "texture" aspect.