Phonology

Phonology is the study of the organization of sounds in a language. It focuses on the unconscious knowledge a speaker has of what sounds are possible in their language (inventory) and how those sounds can be sequenced (phonotactics).

Inventory

For example, English has an aspirated bilabial stop [p], but Spanish does not. On the other hand, Spanish has a voiced bilabial fricative [β], but English does not.

Phonotactics

For example, Words in English can begin with an [s] followed by another consonant, e.g. [spay] "spy". In Spanish, such words are impossible.

Phonetics

Phonetics is not phonology.