For questions about the "vertical" aspects of music and how to build and study them.
Harmony is the musical effect observed when two or more pitches interact in the same perceived time element. Harmony looks at the sum of all notes and how those notes interact with each other. Within Western contexts, the main branch of studying harmony is called diatonic harmony, which is based off of concepts from the Common Practice period.
Harmony can exist in varying degrees of complexity, going far beyond simple diatonic harmony all the way to abstract chromaticism.
Chords - groups of three or more notes sounded simultaneously - are a useful concept to study harmony, and the relationship between chords can be analyzed to study the ebb and flow of music. Musicians often analyze using structural concepts like chords even if the original composer predates the ideas used to study their works.
Harmony is often considered to be one of the main components of music, often alongside or contrasted with rhythm. However, neither is strictly necessary for sound to be considered music, and the inclusion or exclusion of harmony should not be considered a dividing line between music and nonmusic.