Phonetics is the study of sounds made by the human voice, specifically in speech. Therefore, phonetics looks at how sounds are made, what affects their production, etc. A particular language is not the necessarily the focus of phonetics, but rather the sounds produced by a speaker.
Phonology is a branch of linguistics that studies how sounds are used in a language. This branch can include phonetics in its studies.
Specifically, phonetics studies the sounds of speech of any language and phonology studies the patterns of speech within and/or between language(s). Phonetics forms the basis for phonology and phonology is the basis for further studies in other aspects of speech such as morphology, syntax, discourse, etc. Phonology uses phonetics to interpret perceptions made by speakers and listeners in relation to the sounds and how those sounds are significant in speech.