Sectarian is a word to describe something having to do with smaller groups or sects. Sectarian violence, for example, would describe violence that has erupted between conflicting groups or sects.
Sectarian, as a noun, is a member of a group with a particular set of interests. If you're a religious sectarian, you are loyal to a particular religious sect or group. Unfortunately, there are not many places in the world where sectarian strife and conflict do not exist.