What is the difference between phonetics and phonology?
Phonetics studies speech sounds as physical events; phonology studies how a particular language organises them into a system. One is about acoustics and articulation, the other about which differences a language treats as meaningful.
Phonetics is concerned with sounds as they actually are, independent of any language:
Articulatory phonetics — how sounds are produced: where the tongue is, whether the vocal folds vibrate, whether air escapes through the nose.
Acoustic phonetics — the physical properties of the sound wave.
Auditory phonetics — how sounds are perceived.
It uses the International Phonetic Alphabet (IPA), in which each symbol represents one sound, so any human speech sound can be transcribed unambiguously — which ordinary spelling cannot do.
Phonology asks a different question: which sound differences matter in this language?
The phoneme is the central concept — the smallest unit that can change meaning. In English, /p/ and /b/ are separate phonemes because pat and bat are different words. Such a pair is a minimal pair, and it is the standard test.
Allophones are variants of the same phoneme that never change meaning. English speakers produce a puff of air on the p in pin but not in spin — a genuine, measurable phonetic difference that English treats as the same sound, and which most speakers cannot hear until it is pointed out.
Why the distinction matters. The same physical difference can be phonemic in one language and allophonic in another. Aspiration distinguishes words in some languages; English speakers learning them must learn to hear a difference they have been trained since infancy to ignore. Conversely, distinctions English makes are inaudible to speakers of languages that do not.
This is why adult language learning is hard at the sound level — the difficulty is perceptual before it is articulatory.
Phonology also covers syllable structure, stress, intonation, and phonotactics — the rules about which sound sequences are permitted, which is why some words feel impossible in English and unremarkable in Polish.
Brackets signal which is meant: square brackets for phonetic transcription, slashes for phonemic.