What is phonology?
Phonology is the study of natural language (not constructed or formal language), with particular focus on how sounds of natural language are organized. Phonology includes in its study a taking of an inventory of sounds typical of a specific language and the rules by which sounds interact with one another (in regards to significance and interpretation by the natural speaker).
While similar to phonetics, phonology differs in regards to the depth of study. Phonology dissects the morphology (Greek for "study of the shape"--roots, prefixes, suffixes), syntax (Greek for "arrangement"--sentence construction), and discourse (Latin for "running to and from"--as seen in spoken and written communications).