The sounds of a language are organized into a system of contrasts, which are analyzed in terms of phonological units (phonemes, distinctive features) Phonology deals with the range and function of sounds in specific languages, and with the rules that determine the types of phonetic relationship that relate and contrast these linguistic units.
Phoneme theory
In human language, a phoneme is the theoretical representation of a sound. It is a sound of language represented (or imagined) without reference to its position in a word or a phrase. A phoneme, therefore, is the conception of a sound in the most neutral form possible and distinguishes between different words or morphemes. Changing an element of a word from one phoneme another produces either a different word or obvious nonsense.