What is the difference between a phonetician and a phonologist? I've seen these two terms somewhere on this site but can't figure out the difference.
The difference resides in the subject matter that they investigate. They have in common the fact that they investigate language sound systems. Phonology specifically looks at sound from the perspective of computations involving discrete units (including sets of individual units), such as [p] or [p,b,m], and phonetics involves the mapping between symbolic sound and physical sound. So while phonology would describe [p] as e.g. a "bilabial voiceless stop" or [+anterior,-coronal,-voice] depending on your theoretical framework, phonetics would describe the sound in terms of a particular pattern of duration in milliseconds, formant transitions, amplitudes, as well as articulatory movements; also, phonetics would study how incoming continuous sound waves are interpreted as discrete sounds.
There is a high degree of interdependency between these disciplines, so a phonological analysis is based on prior phonetic work that allows us to make a rough mapping between continuous sound and individual segments like [p]. Likewise, a phonetic analysis generally looks at properties of classes of discrete sounds, for example the way that formant transitions differ between [p,b,m] versus [t,d,n], where you need a prior phonological analysis of a language to decide that a certain part of the speech stream is a "p" or a "t".