Phonology is the study of how sounds are organized and used in natural languages.
The phonological system of a language includes
- an inventory of sounds and their features, and
- rules which specify how sounds interact with each other.
Phonology is just one of several aspects of language. It is related to other aspects such as phonetics, morphology, syntax, and pragmatics.
Here is an illustration that shows the place of phonology in an interacting hierarchy of levels in linguistics: