Elementary Music Theory

You experience some sounds as interesting, beautiful, pleasing — or even annoying and disturbing — because music is organized sound (and silence) over time.
"Organized" means there are differences in pitch (how high or low a sound is), and the sounds are arranged with rhythm (when they happen and how long they last). To review:

1. Music — organized sound and silence over time.
2. Rhythm — when and how long sounds (or notes) are played.
3. Interval — the distance in pitch between two notes.
4. Melody — a sequence of notes of different pitches (with rhythm) played over time.
5. Pitch — how high or low a sound is (not how loud — loudness is a separate quality called dynamics or volume).
6. Note — a single musical sound (with pitch and duration).

Intervals are one of the most important building blocks in music theory — they form the basis of scales and chords.
A scale is like a palette: a specific set of notes arranged from low to high with fixed intervals between them — the notes you mostly use in a piece.
A chord is two or more notes of different pitch played at the same time (they often sound harmonious, but not always). Melody + chords (plus rhythm) form the core of most music we know.