A typology is a way of dividing a big group of things into smaller groups, where each group is held together by qualities its members share. The word covers two things at once: the finished system of groups ('he proposed a typology of family structures' — here it is a countable noun, so 'a typology', 'three typologies'), and the activity of studying things this way ('typology is central to modern linguistics' — uncountable). It is academic language. Linguists build typologies of languages, archaeologists of pottery and stone tools, sociologists of personalities and institutions, architects of building shapes. What makes a typology different from a plain list is that the groups are meant to be useful — they should let you predict something about a new case once you know which group it belongs to. Worth knowing: careful scholars distinguish a typology (groups made up in the mind first, then things sorted into them) from a taxonomy (groups discovered from the real differences between things, as in biology), though in everyday academic speech the two words often blur. In theology it has the separate sense of reading earlier scripture as a foreshadowing of later events.