Phonetics relates to the sounds of language, while phonology studies how those sounds are put together to create meaning. Phonemes, or units of sound that are used in all languages to create words, are the focus of the study of phonetics. Phonology studies the rules in any given language that govern how those phonemes are combined to create meaningful words. Phonetics and phonology study two different aspects of sound, but the concepts are dependent on each other in the creation of language.
Each unit of sound, regardless of language, is called a phoneme. Phonetics attempts to understand how each one of these phonemes is physically formed and produced by humans. These units can be categorized by how they are produced and whether they are voiced or voiceless. This aspect of phonetics is commonly used by linguists as well as speech specialists to understand how humans create speech sounds and why speech problems sometimes occur.
The phonetic alphabet is a collection of symbols meant to represent the actual sound of each phoneme as it is pronounced in different situations. For example, a consonant may have two different symbols because it has two different ways of being pronounced depending on the word it is used in. These phonetic symbols are commonly found in dictionaries as a guide to how to correctly pronounce unfamiliar words. Each symbol can represent one letter or a group of letters that combine to make one sound.
Phonology is the study of how phonemes are put together and how they create meaning for the speaker of any given language. Some phonemes may have slightly different meanings or uses in two different languages, and phonology is an attempt to understand these changes in meaning. In addition, historical or diachronic phonology studies how the phonemes of a word can change over time and how this affects word meaning. Phonology also examines the patterns of how phonemes are used in a language. For example, some of these units are only used in the middle or at the end of a word but never at the beginning.
Phonetics and phonology differ in that phonetics studies the production of sounds, and phonology studies the combination of sounds. Phonetics can be used to explore the sounds that are used in any language, but phonology looks at only one language at a time. Both depend on each other because without the production of sounds there would be no words, but without the rules to put them together, sounds would have no meaning. They work together in important ways, but both cover their own specific part of language production.