I'm a linguist (well, I have a degree in linguistics, but I'm not really sure if that makes me a linguist! At any rate, I think linguistics is really exciting! I guess that goes to show how much of a nerd I am), and my favorite branch of linguistics is phonology. (Other branches include syntax, semantics, historical linguistics, psycho- (or neuro-) linguistics, computational linguistics...)
Whenever I mention that I like phonology, people ask, "oh, so is that like phonetics?"
Well, yes and no. Phonetics is a separate branch of linguistics, but it's related to phonology, and you can't do much phonology without phonetics.
Phonetics is the study of sound. That is, the sounds humans make and how we produce them with our voice apparatus (the anatomical parts we use to produce sounds--lungs, vocal cords, tongue, teeth, nose, etc.) Phonetics defines the sounds, how they're made, and has a symbol to represent each distinct sound. Phonology is the study of patterns of sounds, especially the patterns found within a language. (For example, a fairly universal pattern--aka a phonological universal--is that nasals match the place of articulation of the following consonant. This is why we say impossible instead of inpossible--the nasal sound transfers where it is produced with the voice apparatus to match where the next consonant is produced. Since the p is produced with the lips, the nasal that precedes it--in this case the m--matches the place of sound production; the lips.
So, that's the difference between phonetics and phonology! Phonology uses phonetics to study what sound patterns exist, why, and how they work. (By the way, I love phonetics, too! Especially areas like acoustic phonetics and such...)