Phonology is, broadly speaking, the subdiscipline of linguistics
Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....

 concerned with the sounds of language. That is, it is the systematic use of sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 to encode meaning in any spoken human language
Human language
A human language is a language primarily intended for communication among humans. The two major categories of human languages are natural languages and constructed languages...

, or the field of linguistics studying this use. In more narrow terms, "phonology proper is concerned with the function, behaviour and organization of sounds as linguistic items". Just as a language has syntax
In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....

 and vocabulary
A person's vocabulary is the set of words within a language that are familiar to that person. A vocabulary usually develops with age, and serves as a useful and fundamental tool for communication and acquiring knowledge...

, it also has a phonology in the sense of a sound system.