For questions about of how music works, seeking to identify structures and patterns in music.
When talking about theory, most people will think in terms of the Western European tradition during the common practice era. From this came many concepts and ideas that can be seen in music today such as the tonic/dominant relationship, Roman Numeral analysis, and keys. This is by far not the only theory that exists. For example, in post tonal theory looks beyond the methods used in the common practice era such as taking the ideas of dissonance defined by the common practice era and utilizing it to create unique concepts.